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Rollforward of Cumulative Other Than Temporary Credit Losses Recognized in Earnings for Debt Securities (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]      
Estimated credit losses, beginning of year $ 3,246,915 $ 3,154,987 $ 2,892,779
Credit losses - no previous OTTI recognized 0 70,000 0
Credit losses - previous OTTI recognized 0 21,928 262,208
Estimated credit losses, end of year $ 3,246,915 $ 3,246,915 $ 3,154,987