Stock-Based Compensation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Stock-Based Compensation Activity
Income tax benefit	$ 1.0	$ 0.1	$ 2.5	$ 1.9
Performance Shares [Member]
Stock-Based Compensation Activity
Compensation expense	3.9	0.2	9.7	4.7
Restricted Stock [Member]
Stock-Based Compensation Activity
Compensation expense	0.0	0.1	0.0	0.1
Stock Compensation Plan [Member]
Stock-Based Compensation Activity
Compensation expense	3.9	0.3	9.7	4.8
Total Shareholder Return [Member] | Performance Shares [Member]
Stock-Based Compensation Activity
Compensation expense	2.0	1.6	6.1	4.9
Performance Units Related to Earnings Per Share [Member] | Performance Shares [Member]
Stock-Based Compensation Activity
Compensation expense	$ 1.9	$ (1.4)	$ 3.6	$ (0.2)