Goodwill and Other Intangible Assets (Details) - Other Intangible Assets - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Other intangible assets by major asset class
Intangible assets subject to amortization, gross carrying amount	$ 308		$ 308		$ 286
Intangible assets subject to amortization, accumulated amortization	178		178		171
Intangible assets subject to amortization, net	130		130		115
Intangible assets not subject to amortization	226		226		227
Total other intangible assets, gross carrying amount	534		534		513
Total other intangible assets, net	356		356		342
Amortization expense of intangible assets	4	$ 2	8	$ 5
Estimated intangible asset amortization expense, remainder of 2018	8		8
Estimated intangible asset amortization expense, 2019	16		16
Estimated intangible asset amortization expense, 2020	15		15
Estimated intangible asset amortization expense, 2021	14		14
Estimated intangible asset amortization expense, 2022	13		13
Customer-related [Member]
Other intangible assets by major asset class
Intangible assets subject to amortization, gross carrying amount	100		100		77
Intangible assets subject to amortization, accumulated amortization	7		7		4
Intangible assets subject to amortization, net	93		93		73
Contract-based [Member]
Other intangible assets by major asset class
Intangible assets subject to amortization, gross carrying amount	208		208		209
Intangible assets subject to amortization, accumulated amortization	171		171		167
Intangible assets subject to amortization, net	$ 37		$ 37		$ 42