Consolidated Statement of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Common stock	Retained earnings	Accumulated other comprehensive income (loss)	ASU 2018-02 Retained earnings
Beginning Balance at Dec. 31, 2016	$ 982.6	$ 296.1	$ 694.6	$ (8.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	72.3		72.3
Other comprehensive income, net of tax expense	9.1			9.1
Common stock transactions:
Common shares purchased and retired	(0.9)	(0.9)
Stock Issued During Period, Value, New Issues	385.9	385.9
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	2.0	2.0
Stock-based compensation expense	3.4	3.4
Common cash dividend declared	(35.0)		(35.0)
Ending Balance at Sep. 30, 2017	1,419.4	686.5	731.9	1.0
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				(3.1)	$ 3.1
Other comprehensive income, net of tax expense	(24.3)			(24.3)
Common stock transactions:
Common shares purchased and retired	(1.0)	(1.0)
Stock Issued During Period, Value, New Issues	173.3	173.3
Stock options exercised, net of shares 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)
Ending Balance at Sep. 30, 2018	$ 1,654.4	$ 865.5	$ 828.3	$ (39.4)