Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Selected Quarterly Financial Information [Abstract]
Revenues	$ 8,539		$ 8,549		$ 8,247		$ 8,012		$ 8,071		$ 8,107		$ 7,975		$ 8,027		$ 33,347	$ 32,180	$ 34,532
Operating income (loss)	470		311		622		361		8		(197)		(2)		501		1,764	310	(1,895)
Net loss	$ (283)		$ (479)		$ (142)		$ (302)		$ (554)		$ (836)		$ (585)		$ (20)		$ (1,206)	$ (1,995)	$ (3,345)
Basic and diluted net loss per common share	$ (0.07)	[1]	$ (0.12)	[1]	$ (0.04)	[1]	$ (0.08)	[1]	$ (0.14)	[1]	$ (0.21)	[1]	$ (0.15)	[1]	$ (0.01)	[1]	$ (0.30)	$ (0.50)	$ (0.85)

[1] (1)The sum of the quarterly earnings per share amounts may not equal the annual amounts because of the changes in the weighted average number of shares outstanding during the year.