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Rollforward of Cumulative Other Than Temporary Credit Losses Recognized in Earnings for Debt Securities (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]    
Estimated credit losses, beginning of year $ 3,247 $ 3,247
Credit losses - no previous OTTI recognized 0 0
Credit losses - previous OTTI recognized 0 0
Estimated credit losses, end of period $ 3,247 $ 3,247