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Investments - Summary of Amounts Included in Net Realized Gains and Other-Than-Temporary Impairments (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Investments Debt And Equity Securities [Abstract]    
Gross realized gains on securities $ 41,578 $ 824
Gross realized losses on securities (318) (321)
Foreign exchange gains 4,800 (111)
Net realized gains 46,060 392
Net other-than-temporary impairments   $ (3,071)