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	79.5		79.5
Other comprehensive income (loss), net of tax expense	35.1	0.0	0.0	35.1
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	0.7	0.7	0.0	0.0
Stock-based compensation expense	4.6	4.6	0.0	0.0
Common cash dividend declared	(38.8)	0.0	(38.8)	0.0
Equity, ending balance at Jun. 30, 2019	1,948.6	1,045.6	892.5	10.5
Equity, beginning balance at Mar. 31, 2019	1,736.9	866.6	874.7	(4.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	37.9		37.9
Other comprehensive income (loss), net of tax expense	14.9			14.9
Common stock transactions:
Common shares purchased and retired	(0.1)	(0.1)
Stock Issued During Period, Value, New Issues	176.1	176.1
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	2.7	2.7
Common cash dividend declared	(20.1)		(20.1)
Equity, ending balance at Jun. 30, 2019	1,948.6	1,045.6	892.5	10.5
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	66.0		66.0
Other comprehensive income (loss), net of tax expense	54.2	0.0	0.0	54.2
Common stock transactions:
Common shares purchased and retired	(32.6)	(32.6)	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	0.7	0.7	0.0	0.0
Stock-based compensation expense	3.8	3.8	0.0	0.0
Common cash dividend declared	(83.0)	0.0	(83.0)	0.0
Equity, ending balance at Jun. 30, 2020	1,998.9	1,021.2	912.5	65.2
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,989.8	1,049.3	929.5	11.0
Equity, beginning balance at Mar. 31, 2020	1,956.1	1,018.7	897.6	39.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	36.7		36.7
Other comprehensive income (loss), net of tax expense	25.4	0.0	0.0	25.4
Common stock transactions:
Common shares purchased and retired	0.0	0.0	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	0.1	0.1	0.0	0.0
Stock-based compensation expense	2.4	2.4	0.0	0.0
Common cash dividend declared	(21.8)	0.0	(21.8)	0.0
Equity, ending balance at Jun. 30, 2020	$ 1,998.9	$ 1,021.2	$ 912.5	$ 65.2