Summary of Significant Accounting Policies - Schedule of Amounts Charged to Expense for the Depreciation and Amortization of Property and Equipment, Including Equipment under Capital Lease (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Depreciation and amortization of property and equipment, including equipment under capital lease	$ 300	$ 290	$ 286