Stock-Based Compensation - Stock option activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Options
Outstanding at beginning of period, Options	114		130	130
Forfeited, Options	(8)		(16)	0
Expired, Options	(62)		0	0
Outstanding at end of period, Options	44	[1]	114	130
Weighted Average Exercise Price
Outstanding at beginning of period, Weighted Average Exercise Price	$ 217.24		$ 218.46	$ 218.46
Forfeited, Weighted Average Exercise Price	200.95		227.19	0.00
Expired, Weighted Average Exercise Price	193.13		0.00	0.00
Outstanding at end of period, Weighted Average Exercise Price	$ 243.26	[1]	$ 217.24	$ 218.46
Stock option activity, additional disclosure
Options exercisable at end of period, Options	22	[2]	85	93
Options exercisable at end of period, Weighted Average Exercise Price	$ 228.81	[2]	$ 203.48	$ 202.48
Available for grant at end of period	16		26	10
Stock options outstanding, weighted average remaining contractual term	3 years 8 months 24 days
Stock options outstanding, intrinsic value	$ 0.0
Exercisable stock options, weighted average remaining contractual term	3 years 1 month 24 days
Stock options exercisable, intrinsic value	$ 0.0

[1] Stock options outstanding at December 31, 2017 have a weighted average remaining contractual term of 3.7 years and there is no intrinsic value.

[2] Exercisable stock options at December 31, 2017 have a weighted average remaining contractual term of 3.1 years and there is no intrinsic value.