Stock-Based Compensation - Restricted Stock Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Stock-based compensation expense	$ 16.4	$ 14.7	$ 14.2
Stock-based compensation expense for retirement savings plan contributions	2.7	2.5	2.4
Associated current and future income tax benefit recognized	3.3	$ 4.6	$ 4.3
Unrecognized compensation cost related to non-vested share-based awards	$ 22.2
Unrecognized compensation cost, weighted-average period for recognition	2 years
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Restricted stock awards outstanding at beginning of period (in shares)	1,436
Awards granted (in shares)	747
Awards vested (in shares)	(669)
Awards forfeited (in shares)	(104)
Restricted stock awards outstanding at end of period (in shares)	1,410	1,436
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Awards outstanding at beginning of period (in dollars per share)	$ 18.12
Awards granted (in dollars per share)	23.62	$ 18.68	$ 16.79
Awards vested (in dollars per share)	18.17
Awards forfeited (in dollars per share)	17.68
Awards outstanding at end of period (in dollars per share)	20.95	18.12
Awards granted (in dollars per share)	$ 23.62	$ 18.68	$ 16.79
Fair value of shares vested	$ 15.8	$ 12.0	$ 10.8
Restricted Stock | Minimum
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Requisite service period	3 years