Stockholders' Equity - Stock Options Exercise Data (Details) - Stock Options - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of stock option exercise data
Number of options exercised (in shares)	243,624	985,583	1,462,369
Cash received from options exercised	$ 7.7	$ 39.3	$ 50.2
Total intrinsic value of options exercised	21.9	120.3	108.4
Tax benefits realized from options exercised	$ 5.9	$ 37.4	$ 30.8