Commitments and Contingencies, Letters of Credit and Guarantees (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) lease_agreement	Dec. 31, 2017 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Total outstanding letters of credit	$ 410	$ 416
Available borrowings under Credit Facility	$ 1,000
Number of existing real estate lease agreements | lease_agreement	3
Real estate lease agreements, expiry date	Aug. 31, 2020
Purchase price of property on or before expiration date	$ 45
Residual guarantee amount	39
Maximum amount of supplemental rent payment	6
Aggregate residual guarantee amount	$ 39