Leases - Narrative (Details) - USD ($) $ in Millions		3 Months Ended	6 Months Ended
	Jan. 01, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Leases [Abstract]
Additional lease liabilities	$ 1,200		$ 1,095		$ 1,149
Right-of-use asset	1,000		930
Deferred tax assets	252
Deferred tax liabilities	252
Adjustment of retained earnings	$ 0
Rent expense, net of sublease income		$ 64		$ 128
Non-cash additions to operating lease ROU assets			$ 13