Fair Value of Financial Assets and Liabilities Contractual Maturities of Investments (Detail) (Debt Securities, USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Debt Securities
Amortized Cost Basis
Due within one year	$ 1,220,229
Due after 1 year through 5 years	4,423,210
Due after 5 years through 10 years	484,206
Due after 10 years	409,321
Available For Sale Securities Amortized Cost	6,536,966	6,534,797
Aggregate Fair Value
Due within one year	1,223,463
Due after 1 year through 5 years	4,450,632
Due after 5 years through 10 years	511,333
Due after 10 years	408,256
Available For Sale Securities Fair Value	$ 6,593,684	$ 6,595,090

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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