Contractual Maturities of Investments (Detail) (Debt Securities, USD $)	Mar. 31, 2012	Dec. 31, 2011
Debt Securities
Amortized Cost Basis
Due within one year	$ 1,658,223,000
Due after 1 year through 5 years	3,654,373,000
Due after 5 years through 10 years	440,857,000
Due after 10 years	330,787,000
Available For Sale Securities Amortized Cost	6,084,240,000	6,228,479,000
Aggregate Fair Value
Due within one year	1,662,038,000
Due after 1 year through 5 years	3,677,387,000
Due after 5 years through 10 years	456,844,000
Due after 10 years	329,530,000
Available For Sale Securities Fair Value	$ 6,125,799,000	$ 6,247,092,000

For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading.

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