Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based compensation
Compensation expense related to the Plan	$ 16.0	$ 14.8	$ 14.1
Restricted Stock
Stock-based compensation
Period over which forfeiture and non-transferable restrictions lapse	4 years
Percentage over which unrecognized compensation cost is expected to be recognized year one	15.00%
Percentage over which unrecognized compensation cost is expected to be recognized year two	15.00%
Percentage over which unrecognized compensation cost is expected to be recognized year three	20.00%
Percentage over which unrecognized compensation cost is expected to be recognized year four	50.00%
Unrecognized compensation cost related to the restricted stock	$ 22.5
Shares
Balance at the beginning of the period (in shares)	919,193
Granted (in shares)	320,018	316,486	295,148
Vested (in shares)	(328,253)
Forfeited (in shares)	(11,484)
Balance at the end of the period (in shares)	899,474	919,193
Weighted Average Grant-Date Fair value
Balance at the beginning of the period (in dollars per share)	$ 22.73
Granted (in dollars per share)	51.60
Vested (in dollars per share)	47.06
Forfeited (in dollars per share)	44.57
Balance at the end of the period (in dollars per share)	$ 46.59	$ 22.73
Aggregate Intrinsic Value
Balance at the end of the period (in dollars)	$ 43.0
2015 Plan
Stock-based compensation
Plan term	5 years
Maximum number of shares authorized under the plan	4,000,000
Number of shares of common stock available for grant under the plan	3,219,205