The vast majority of SEC XBRL financial filers get the fiscal period focus, fiscal year focus, document period end date, current balance sheet date, and what the SEC EFM calls the "required context" correct. A handful don't synchronize these correctly.
Take the fiscal year. First of, of 7160 SEC filers, 6999 (or 97.7%) identify the fiscal period focus as fiscal year correctly using "FY". But there are 152 who incorrectly using "Q4" or "Q3", "Q2", "CY", and "T2". Using FY is the only correct option for a 10-K.
In terms of the fiscal year focus, 6987 (or 97.5%) properly syncronize the fiscal year focus and the current balance sheet date. But there are 173 (or 2.5%) who do not. Here is an example of an error to help you understand this:
Consider this document information:
Here is the balance sheet for the same submission:
Notice how the current balance sheet period "Dec 31, 2011", the Document Period End Date in the document information of "Dec 31, 2011" and the end date of the context for the column with the document period end date in it of "Dec 31, 2011" are all the same. That is the way that information works in the vast majority of SEC XBRL financial filings.
But look at the fiscal year focus in the document information which has a value of "2012". Also see that the current fiscal year end date is --12-31 (although it is partially obscured by the red circle).
Clearly something is wrong here. All the dates would indicate that the fiscal year is 2011 and not the 2012 reported by this filer.
Also consider the Edgar Filer Manual section 6.5.19:
If you read that section you can see that you have some other dates to deal with. While the dei:DocumentPeriodEndDate specifies the current balance sheet date, so does the end date of the context of what the SEC refers to as the "Required Context". The fact which reports the dei:DocumentPeriodEndDate will have that required context. The end date of that required context will match (a) the dei:DocumentPeriodEndDate and (b) the current balance sheet date. This is what that looks like in the form of XML:
If you look close, the end date matches. Now, of 7160 SEC filings, 7100 (or 99.1%) this does match and for 60 filings it does NOT match.
For the income statement same deal. There were roughly the same number of filings where the income statement dates matched that required context start date and end date. However, this is a little more complicated to check because developing stage companies report additional periods. So, I cannot give a precise number.
Getting these dates correct and correctly synchronized is important because it is these dates which are used to identify the correct information from within a filing and for extract information from the SEC XBRL financial filing.