Share-Based Compensation and Capital Stock - Pre-Tax Stock-Based Compensation Expenses And Associated Income Tax Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-Based Compensation and Capital Stock [Line Items]
Share-based compensation expense	$ 4.0	$ 3.7	$ 7.7	$ 7.2
Income tax benefits	1.0	0.9	1.9	1.7
Stock options
Share-Based Compensation and Capital Stock [Line Items]
Share-based compensation expense	0.5	0.6	1.1	1.3
Restricted stock
Share-Based Compensation and Capital Stock [Line Items]
Share-based compensation expense	2.1	1.6	4.1	3.3
Performance Vested Restricted Stock Units
Share-Based Compensation and Capital Stock [Line Items]
Share-based compensation expense	$ 1.4	$ 1.5	$ 2.5	$ 2.6