Contractual Maturities of Investments (Detail) (Debt Securities, USD $)	Dec. 31, 2011
Debt Securities
Amortized Cost Basis
Due within one year	$ 1,777,170,000
Due after 1 year through 5 years	3,697,240,000
Due after 5 years through 10 years	443,037,000
Due after 10 years	311,032,000
Total	6,228,479,000
Aggregate Fair Value
Due within one year	1,780,391,000
Due after 1 year through 5 years	3,714,177,000
Due after 5 years through 10 years	448,203,000
Due after 10 years	304,321,000
Total	$ 6,247,092,000