Share-Based Compensation - Stock Options Exercise Data (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Summary of stock option exercise data
Cash received from options exercised	$ 8.8	$ 9.2
Stock Options
Summary of stock option exercise data
Number of options exercised (in shares)	166,508	174,295
Cash received from options exercised	$ 8.8	$ 9.2
Total intrinsic value of options exercised	$ 10.3	$ 10.4