Stockholders' Equity - Stock Options Exercise Data (Details) - Stock Options - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Summary of stock option exercise data
Number of options exercised (in shares)	40,565	42,443	428,541	196,984
Cash received	$ 1.6	$ 1.2	$ 38.2	$ 6.2
Total intrinsic value of options exercised	$ 3.5	$ 3.9	$ 27.0	$ 17.3