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	41.6		41.6
Other comprehensive income (loss), net of tax expense	20.2			20.2
Common stock transactions:
Common shares purchased and retired	(2.4)	(2.4)
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	0.4	0.4
Stock-based compensation expense	1.9	1.9
Common cash dividend declared	(18.7)		(18.7)
Equity, ending balance at Mar. 31, 2019	1,736.9	866.6	874.7	(4.4)
Income Tax Effects Allocated Directly to Equity, Cumulative Effect of Change in Accounting Principle	(24.1)		(24.1)
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	29.3		29.3
Other comprehensive income (loss), net of tax expense	28.8	0.0	0.0	28.8
Common stock transactions:
Common shares purchased and retired	(32.6)	(32.6)	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	0.6	0.6	0.0	0.0
Stock-based compensation expense	1.4	1.4	0.0	0.0
Common cash dividend declared	(61.2)	0.0	61.2	0.0
Equity, ending balance at Mar. 31, 2020	1,956.1	1,018.7	897.6	39.8
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,989.8	$ 1,049.3	$ 929.5	$ 11.0