Employee Stock and Incentive Plans - Non-Qualified Stock Options (Details) - Non-Qualified Stock Options [Member] - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Options [Roll Forward]
Outstanding as of January 1	4,357	39,654	66,279
Exercised	(4,357)	(35,297)	(24,456)
Forfeited	0	0	(2,169)
Outstanding as of December 31	0	4,357	39,654
Exercisable as of December 31	0	4,357	39,654
Weighted-Average Exercise Price [Abstract]
Outstanding as of January 1	$ 40.29	$ 44.39	$ 44.39
Exercised	40.29	44.89	44.52
Forfeited	0.00	0	42.93
Outstanding as of December 31	0.00	40.29	44.39
Exercisable as of December 31	$ 0.00	$ 40.29	$ 44.39
Cash Received from Non-Qualified Stock Options Exercised	$ 0.3
Total Intrinsic Value of Options Exercised	$ 0.1	$ 0.5	$ 0.2