Fair Value of Financial Assets and Liabilities - Contractual Maturities of Investments (Detail) (Fixed income securities, USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fixed income securities
Amortized Cost Basis
Due within one year	$ 2,036
Due after 1 year through 5 years	5,905
Due after 5 years through 10 years	524
Due after 10 years	320
Amortized Cost	8,785	8,115
Aggregate Fair Value
Due within one year	2,037
Due after 1 year through 5 years	5,920
Due after 5 years through 10 years	523
Due after 10 years	319
Aggregate Fair Value	$ 8,799	$ 8,103