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, 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	0.0	0.0	38.0
Common stock transactions:
Common shares purchased and retired	(2.5)	(2.5)	0.0	0.0
Stock Issued During Period, Value, New Issues	176.1	176.1	0.0	0.0
Non-vested common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0	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	0.0	0.0
Stock-based compensation expense	6.6	6.6	0.0	0.0
Common cash dividend declared	(59.0)	0.0	(59.0)	0.0
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 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
Income Tax Effects Allocated Directly to Equity, Cumulative Effect of Change in Accounting Principle	(24.1)	0.0	(24.1)	0.0
Equity, beginning balance at Dec. 31, 2019	2,013.9	1,049.3	953.6	11.0
Equity, beginning balance at Dec. 31, 2019	2,013.9	1,049.3	953.6	11.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	114.3		114.3
Other comprehensive income (loss), net of tax expense	50.9	0.0	0.0	50.9
Common stock transactions:
Common shares purchased and retired	(78.9)	(78.9)	0.0	0.0
Stock Issued During Period, Value, New Issues	0.0	0.0	0.0	0.0
Non-vested common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0	0.0	0.0
Stock options exercised, net of share tendered in payment of option price and income tax withholding amounts	0.8	0.8	0.0	0.0
Stock-based compensation expense	5.6	5.6	0.0	0.0
Common cash dividend declared	(104.9)	0.0	(104.9)	0.0
Equity, ending balance at Sep. 30, 2020	1,977.6	976.8	938.9	61.9
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,989.8	1,049.3	929.5	11.0
Equity, beginning balance at Jun. 30, 2020	1,998.9	1,021.2	912.5	65.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	48.3		48.3
Other comprehensive income (loss), net of tax expense	(3.3)	0.0	0.0	(3.3)
Common stock transactions:
Common shares purchased and retired	(46.3)	(46.3)	0.0	0.0
Non-vested common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0	0.0	0.0
Stock options exercised, net of share tendered in payment of option price and income tax withholding amounts	0.1	0.1	0.0	0.0
Stock-based compensation expense	1.8	1.8	0.0	0.0
Common cash dividend declared	(21.9)	0.0	(21.9)	0.0
Equity, ending balance at Sep. 30, 2020	$ 1,977.6	$ 976.8	$ 938.9	$ 61.9