Stock-based Compensation - Effects on Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Proceeds from common stock transactions	$ 442	$ 616	$ 1,314
Tax benefit realized from stock options exercised	$ 171	$ 218	$ 227