CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Millions	Total	Common stock	Additional paid-in capital	Retained earnings	Retained earnings Cumulative effect of change in accounting principle	Accumulated other comprehensive income (loss), net of tax:	Treasury stock, at cost
Beginning balance at Dec. 31, 2018		$ 0.1	$ 475.1	$ 1,216.6		$ 84.7	$ (485.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 184.4			184.4
Options exercised and conversion of common stock units and restricted stock units			(0.5)
Share-based compensation expense			6.4
Dividends, 2021, $1.24 per share; 2020, $1.20 per share; 2019, $1.15 per share				(48.5)
Change in net unrealized investment gains (losses) on fixed maturity securities	133.5					133.5
Change in net funded status of benefit plans	1.4					1.4
Acquisition of shares							0.0
Ending balance at Dec. 31, 2019	1,567.3	0.1	481.0	1,352.5	$ (0.5)	219.6	(485.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	133.3			133.3
Options exercised and conversion of common stock units and restricted stock units			1.5
Share-based compensation expense			5.9
Dividends, 2021, $1.24 per share; 2020, $1.20 per share; 2019, $1.15 per share				(50.7)
Change in net unrealized investment gains (losses) on fixed maturity securities	135.9					135.9
Change in net funded status of benefit plans	(0.4)					(0.4)
Acquisition of shares							(2.2)
Ending balance at Dec. 31, 2020	1,790.1	0.1	488.4	1,434.6		355.1	(488.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	142.8			142.8
Options exercised and conversion of common stock units and restricted stock units			(0.8)
Share-based compensation expense			7.7
Dividends, 2021, $1.24 per share; 2020, $1.20 per share; 2019, $1.15 per share				(52.5)
Change in net unrealized investment gains (losses) on fixed maturity securities	(75.6)					(75.6)
Change in net funded status of benefit plans	1.0					1.0
Acquisition of shares							(5.3)
Ending balance at Dec. 31, 2021	$ 1,807.4	$ 0.1	$ 495.3	$ 1,524.9		$ 280.5	$ (493.4)