Discontinued Operations - Income Statement (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Expenses
Income from discontinued operations, net of tax	$ (29)	$ (4)	$ 821	$ 40	$ 75	$ 42	$ 35	$ 25	$ 828	$ 177	$ 169
Tempo Business | Discontinued Operations
Revenue
Total revenue									698	2,218	2,202
Expenses
Total operating expenses									656	1,950	1,941
Operating Income from discontinued operations									42	268	261
Other income									10	0	0
Income from discontinued operations before income taxes									52	268	261
Income taxes									3	91	92
Income from discontinued operations excluding gain, net of tax									49	177	169
Gain on sale of discontinued operations, net of tax									779	0	0
Income from discontinued operations, net of tax									$ 828	$ 177	$ 169