Summary of Significant Accounting Policies - Share-based Compensation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period	4 years
Cumulative-effect adjustment from adoption of ASU 2016-09		$ (1,705)
Additional Paid-In Capital
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative-effect adjustment from adoption of ASU 2016-09		$ (39)