Leases - Summary of Future Minimum Lease Receipts and Commitments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Leases [Abstract]
Future minimum lease receipts, direct financing leases, 2019	$ 14
Future minimum lease receipts, direct financing leases, 2020	10
Future minimum lease receipts, direct financing leases, 2021	7
Future minimum lease receipts, direct financing leases, 2022	5
Future minimum lease receipts, direct financing leases, 2023	5
Future minimum lease receipts, direct financing leases, thereafter	19
Future minimum lease receipts, direct financing leases, total	60		$ 77
Future minimum lease receipts, operating leases, 2019	416
Future minimum lease receipts, operating leases, 2020	388
Future minimum lease receipts, operating leases, 2021	360
Future minimum lease receipts, operating leases, 2022	331
Future minimum lease receipts, operating leases, 2023	306
Future minimum lease receipts, operating leases, thereafter	1,704
Future minimum lease receipts, operating leases, total	3,505
Future minimum lease commitments, capital leases, 2019	38
Future minimum lease commitments, capital leases, 2020	36
Future minimum lease commitments, capital leases, 2021	34
Future minimum lease commitments, capital leases, 2022	33
Future minimum lease commitments, capital leases, 2023	30
Future minimum lease commitments, capital leases, thereafter	201
Future minimum lease commitments, capital leases, Total	372
Less amount representing interest	(125)
Present value of minimum capital lease payments	247
Current portion of capital lease obligation	(21)
Long-term portion of capital lease obligation	226	$ 244	$ 244
Future minimum lease commitments, operating leases, 2019	183
Future minimum lease commitments, operating leases, 2020	172
Future minimum lease commitments, operating leases, 2021	158
Future minimum lease commitments, operating leases, 2022	145
Future minimum lease commitments, operating leases, 2023	130
Future minimum lease commitments, operating leases, thereafter	831
Future minimum lease commitments, operating leases, total	1,619
Minimum sublease rentals under non-cancelable subleases	$ 2,290