Common Stock - Stock Options Status and Activity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Shares Under Option
Outstanding, Beginning Balance (in shares)	1,144,719
Adjustment (in shares)	2,200
Exercised (in shares)	(21,392)
Outstanding, Ending Balance (in shares)	1,125,527	1,144,719
Weighted Avg. Exercise Prices
Outstanding, Beginning Balance (in dollars per share)	$ 34.69
Adjustment (in dollars per share)	1.99
Exercised (in dollars per share)	8.56
Outstanding, Ending Balance (in dollars per share)	$ 35.12	$ 34.69
Weighted Avg. Remaining Life in Years	3 years 6 months	3 years 8 months
Aggregate Intrinsic Value, Outstanding, Beginning Balance	$ 19
Aggregate Intrinsic Value, Exercised	1
Aggregate Intrinsic Value, Outstanding, Ending Balance	$ 12	$ 19