Common Stock - Stock Options Status and Activity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Shares Under Option
Outstanding, Beginning Balance (in shares)	313,409	318,016	318,016
Exercised (in shares)		(4,607)
Forfeited (in shares)	(2,368)
Outstanding, Ending Balance (in shares)	311,041	313,409	311,041	318,016
Weighted Avg. Exercise Prices
Outstanding, Beginning Balance (in dollars per share)	$ 43.84	$ 43.60	$ 43.60
Exercised (in dollars per share)		26.78
Forfeited (in dollars per share)	54.34
Outstanding, Ending Balance (in dollars per share)	$ 43.76	$ 43.84	$ 43.76	$ 43.60
Weighted Avg. Remaining Life in Years	1 year 9 months 18 days	2 years 1 month 6 days		2 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding, Beginning Balance	$ 4	$ 5	$ 5
Aggregate Intrinsic Value, Exercised	0	0
Aggregate Intrinsic Value, Outstanding, Ending Balance	$ 2	$ 4	$ 2	$ 5