Consolidated Statement of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Common stock	Retained earnings	Accumulated other comprehensive income (loss)
Equity, beginning balance at Dec. 31, 2017	$ 1,427.6	$ 687.0	$ 752.6	$ (12.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	119.8		119.8
Reclassification of the income tax effects of the Tax Cut and Jobs Act from AOCI	0.0		3.1	(3.1)
Other comprehensive income (loss), net of tax expense	(24.3)			(24.3)
Common stock transactions:
Common shares purchased and retired	(1.0)	(1.0)
Common shares issued	173.3	173.3
Non-vested common shares issued	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0
Stock options exercised, net of share tendered in payment of option price and income tax withholding amounts	1.9	1.9
Stock-based compensation expense	4.3	4.3
Common cash dividend declared	(47.2)		(47.2)
Equity, ending balance at Sep. 30, 2018	1,654.4	865.5	828.3	(39.4)
Equity, beginning balance at Jun. 30, 2018	1,458.3	690.7	802.6	(35.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	41.4		41.4
Other comprehensive income (loss), net of tax expense	(4.4)			(4.4)
Common stock transactions:
Common shares purchased and retired	(0.1)	(0.1)
Common shares issued	173.3	173.3
Non-vested common shares forfeited or canceled	0.0	0.0
Stock options exercised, net of share tendered in payment of option price and income tax withholding amounts	0.3	0.3
Stock-based compensation expense	1.3	1.3
Common cash dividend declared	(15.7)		(15.7)
Equity, ending balance at Sep. 30, 2018	1,654.4	865.5	828.3	(39.4)
Equity, beginning balance at Dec. 31, 2018	1,693.9	866.7	851.8	(24.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	128.6		128.6
Other comprehensive income (loss), net of tax expense	38.0			38.0
Common stock transactions:
Common shares purchased and retired	(2.5)	(2.5)
Common shares issued	176.1	176.1
Non-vested common shares issued	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0
Stock options exercised, net of share tendered in payment of option price and income tax withholding amounts	0.9	0.9
Stock-based compensation expense	6.6	6.6
Common cash dividend declared	(59.0)		(59.0)
Equity, ending balance at Sep. 30, 2019	1,982.6	1,047.8	921.4	13.4
Equity, beginning balance at Jun. 30, 2019	1,948.6	1,045.6	892.5	10.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	49.1		49.1
Other comprehensive income (loss), net of tax expense	2.9			2.9
Common stock transactions:
Common shares purchased and retired	0.0	0.0
Non-vested common shares issued	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0
Stock options exercised, net of share tendered in payment of option price and income tax withholding amounts	0.2	0.2
Stock-based compensation expense	2.0	2.0
Common cash dividend declared	(20.2)		(20.2)
Equity, ending balance at Sep. 30, 2019	$ 1,982.6	$ 1,047.8	$ 921.4	$ 13.4