Consolidated Statement of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Common stock	Retained earnings	Accumulated other comprehensive income (loss)
Beginning Balance at Dec. 31, 2016	$ 982.6	$ 296.1	$ 694.6	$ (8.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	23.2		23.2
Other comprehensive loss, net of tax expense	5.5			5.5
Common stock transactions:
Common shares purchased and retired	(0.9)	(0.9)
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	1.1	1.1
Stock-based compensation expense	0.9	0.9
Common cash dividend declared	(10.8)		(10.8)
Ending Balance at Mar. 31, 2017	1,001.6	297.2	707.0	(2.6)
Beginning Balance at Dec. 31, 2017	1,427.6	687.0	752.6	(12.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	36.7		36.7
Reclassification of the income tax effects of the Tax Cut and Jobs Act from AOCI				(3.1)
Other comprehensive loss, net of tax expense	(17.2)			(17.2)
Common stock transactions:
Common shares purchased and retired	(0.9)	(0.9)
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	0.8	0.8
Stock-based compensation expense	1.1	1.1
Common cash dividend declared	(15.7)		(15.7)
Ending Balance at Mar. 31, 2018	$ 1,432.4	$ 688.0	$ 776.7	$ (32.3)