Common Stock - Stock Options Status and Activity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Shares Under Option
Outstanding, Beginning Balance (in shares)	434,821	438,341	606,525	606,525
Canceled (in shares)			(2,214)
Forfeited (in shares)	(41,369)	(278)	(1,107)
Exercised (in shares)	(50,125)	(3,242)	(164,863)
Outstanding, Ending Balance (in shares)	343,327	434,821	438,341	343,327	606,525
Weighted Avg. Exercise Prices
Outstanding, Beginning Balance (in dollars per share)	$ 40.16	$ 40.22	$ 38.54	$ 38.54
Canceled (in dollars per share)			56.23
Forfeited (in dollars per share)	19.53	66.54	56.23
Exercised (in dollars per share)	34.79	45.42	33.70
Outstanding, Ending Balance (in dollars per share)	$ 43.44	$ 40.16	$ 40.22	$ 43.44	$ 38.54
Weighted Avg. Remaining Life in Years		2 years 5 months 6 days	2 years 7 months 6 days	2 years 5 months 6 days	2 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding, Beginning Balance	$ 8	$ 11	$ 12	$ 12
Aggregate Intrinsic Value, Exercised	1	0	5
Aggregate Intrinsic Value, Outstanding, Ending Balance	$ 4	$ 8	$ 11	$ 4	$ 12