Investments and Marketable Securities - Schedule of Realized Gains on Sales of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Proceeds	$ 25	$ 644	$ 337
Realized gains	0	1	1
Realized losses	0	(1)	0
Net realized gains included in other comprehensive income (loss) as of the end of the prior year	$ 0	$ 0	$ 0