Stockholders' Equity - Stock Options Exercise Data (Details) - Stock Options - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Summary of stock option exercise data
Number of options exercised (in shares)	42,443	210,256	196,984	807,755
Cash received	$ 1.2	$ 8.6	$ 6.2	$ 33.0
Total intrinsic value of options exercised	$ 3.9	$ 25.3	$ 17.3	$ 101.1