Fair Value of Financial Assets and Liabilities Contractual Maturities of Investments (Detail) (Debt Securities, USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Debt Securities
Amortized Cost Basis
Due within one year	$ 1,271,478
Due after 1 year through 5 years	4,083,712
Due after 5 years through 10 years	463,518
Due after 10 years	394,200
Available For Sale Securities Amortized Cost	6,212,908	6,228,479
Aggregate Fair Value
Due within one year	1,275,151
Due after 1 year through 5 years	4,118,038
Due after 5 years through 10 years	488,630
Due after 10 years	392,388
Available For Sale Securities Fair Value	$ 6,274,207	$ 6,247,092

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

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