Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Selected Quarterly Financial Information [Abstract]
Revenues	$ 8,083	$ 8,239	$ 7,927	$ 8,157	$ 8,539	$ 8,549	$ 8,247	$ 8,012	$ 32,406	$ 33,347	$ 32,180
Operating income (loss)	236	727	601	1,163	470	311	622	361	2,727	1,764	310
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	63	7,156	(48)	206
Net Income (Loss) Attributable to Parent	$ 0	$ 0	$ 0	$ 0	$ (283)	$ (479)	$ (142)	$ (302)	$ 7,389	$ (1,206)	$ (1,995)
Earnings Per Share, Basic	$ 0.02	$ 1.79	$ (0.01)	$ 0.05					$ 1.85	$ (0.30)	$ (0.50)
Earnings Per Share, Diluted	$ 20,000	$ 1,760,000	$ (10,000)	$ 50,000					$ 1.81	$ (0.30)	$ (0.50)
Basic and diluted net loss per common share					$ (0.07)	$ (0.12)	$ (0.04)	$ (0.08)