Common Stock - Stock Options Status and Activity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2016
Shares Under Option
Outstanding, Beginning Balance (in shares)	318,016
Exercised (in shares)	(4,607)
Outstanding, Ending Balance (in shares)	313,409
Weighted Avg. Exercise Prices
Outstanding, Beginning Balance (in dollars per share)	$ 43.60
Exercised (in dollars per share)	26.78
Outstanding, Ending Balance (in dollars per share)	$ 43.84
Weighted Avg. Remaining Life in Years	2 years 1 month 6 days	2 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding, Beginning Balance	$ 5
Aggregate Intrinsic Value, Exercised	0
Aggregate Intrinsic Value, Outstanding, Ending Balance	$ 4