Significant Accounting Policies - Discontinued Operations and Earnings per Common Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended	12 Months Ended
Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Accounting Policies
Income (loss) from discontinued operations				$ (13)	$ (2)	$ 15
Income tax expense				5	1	(6)
Income (loss) from discontinued operations, net of tax	$ 1	$ (9)	$ (1)	$ (8)	$ (1)	$ 9
Antidilutive securities excluded from computation of earnings per share (in shares)				10.4	6.7	2.7

Amount of tax expense (benefit) related to a discontinued operation. Includes, but is not limited to, tax expense (benefit) related to income (loss) from operations during the phase-out period, tax expense (benefit) related to gain (loss) on disposal, tax expense (benefit) related to gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and tax expense (benefit) related to adjustments of a prior period gain (loss) on disposal.

Amount after tax of income (loss) from a discontinued operation including the portion attributable to the noncontrolling interest. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

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