Share-Based Compensation Performance Unit Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted-average grant date fair value of other-than-options additional disclosures [Rollforward]
Non-vested, beginning balance	$ 0	$ 0
Non-vested, ending balance	$ 0	$ 0	$ 0
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures	$ 84	$ 78	$ 91
Performance units [Member]
Non-vested equity instruments other than options [Roll Forward]
Non-vested, beginning balance	1,932
Granted	3,070
Vested	(145)
Forfeited	(1,767)
Non-vested, ending balance	3,090	1,932
Weighted-average grant date fair value of other-than-options additional disclosures [Rollforward]
Non-vested, beginning balance	$ 7.08
Granted	6.06	$ 5.11	$ 10.45
Vested	6.25
Forfeited	7.15
Non-vested, ending balance	$ 6.07	$ 7.08
Equity Awards Granted, Vested and Exercised	145
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures	$ 1