Fair Value of Financial Assets and Liabilities - Contractual Maturities of Investments (Detail) (Fixed income securities, USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fixed income securities
Amortized Cost Basis
Due within one year	$ 2,039
Due after 1 year through 5 years	5,974
Due after 5 years through 10 years	550
Due after 10 years	419
Amortized Cost	8,982	9,576
Aggregate Fair Value
Due within one year	2,041
Due after 1 year through 5 years	5,978
Due after 5 years through 10 years	550
Due after 10 years	416
Aggregate Fair Value	$ 8,985	$ 9,602