CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Common stock	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income (loss), net of tax	Treasury stock, at cost	Cumulative Effect, Period of Adoption, Adjustment Retained earnings
Beginning balance at Dec. 31, 2019		$ 0.1	$ 481.0	$ 1,352.5	$ 219.7	$ (485.9)	$ (0.5)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common stock units and restricted stock units			1.6
Share-based compensation expense			4.2
Net income	$ 85.5			85.5
Dividends, 2021, $0.31, $0.93 per share; 2020, $0.30, $0.90 per share				(38.0)
Change in net unrealized investment gains (losses) on fixed maturity securities					97.8
Acquisition of shares	(2.2)
Ending balance at Sep. 30, 2020	1,715.8	0.1	486.8	1,399.5	317.5	(488.1)
Beginning balance at Jun. 30, 2020		0.1	483.8	1,375.7	268.4	(488.1)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common stock units and restricted stock units			1.3
Share-based compensation expense			1.7
Net income	36.5			36.5
Dividends, 2021, $0.31, $0.93 per share; 2020, $0.30, $0.90 per share				(12.7)
Change in net unrealized investment gains (losses) on fixed maturity securities					49.1
Ending balance at Sep. 30, 2020	1,715.8	0.1	486.8	1,399.5	317.5	(488.1)
Beginning balance at Dec. 31, 2020	1,790.1	0.1	488.4	1,434.6	355.1	(488.1)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common stock units and restricted stock units			(1.0)
Share-based compensation expense			5.5
Net income	102.3			102.3
Dividends, 2021, $0.31, $0.93 per share; 2020, $0.30, $0.90 per share				(39.4)
Change in net unrealized investment gains (losses) on fixed maturity securities					(59.4)
Acquisition of shares	(1.7)
Ending balance at Sep. 30, 2021	1,796.4	0.1	492.9	1,497.5	295.7	(489.8)
Beginning balance at Jun. 30, 2021		0.1	490.7	1,494.4	321.0	(489.6)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common stock units and restricted stock units			0.2
Share-based compensation expense			2.0
Net income	16.3			16.3
Dividends, 2021, $0.31, $0.93 per share; 2020, $0.30, $0.90 per share				(13.2)
Change in net unrealized investment gains (losses) on fixed maturity securities					(25.3)
Acquisition of shares	(0.2)
Ending balance at Sep. 30, 2021	$ 1,796.4	$ 0.1	$ 492.9	$ 1,497.5	$ 295.7	$ (489.8)