Fair Value of Financial Assets and Liabilities - Contractual Maturities of Investments (Detail) (Total fixed income securities, USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Total fixed income securities
Amortized Cost Basis
Due within one year	$ 1,404,872
Due after 1 year through 5 years	4,222,054
Due after 5 years through 10 years	466,061
Due after 10 years	401,906
Amortized Cost Basis	6,494,893	6,228,479
Aggregate Fair Value
Due within one year	1,407,373
Due after 1 year through 5 years	4,251,311
Due after 5 years through 10 years	493,258
Due after 10 years	403,244
Aggregate Fair Value	$ 6,555,186	$ 6,247,092