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 [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units			0.2
Share-based compensation expense			2.5
Net income	$ 49.0			49.0
Dividends, 2021, $0.31, $0.62 per share; 2020, $0.30, $0.60 per share				(25.3)
Change in net unrealized investment gains (losses) on fixed maturity securities					48.7
Acquisition of shares	(2.2)
Ending balance at Jun. 30, 2020	1,639.8	0.1	483.7	1,375.7	268.4	(488.1)
Beginning balance at Mar. 31, 2020		0.1	481.9	1,357.8	125.9	(488.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units			0.4
Share-based compensation expense			1.4
Net income	30.5			30.5
Dividends, 2021, $0.31, $0.62 per share; 2020, $0.30, $0.60 per share				(12.6)
Change in net unrealized investment gains (losses) on fixed maturity securities					142.5
Ending balance at Jun. 30, 2020	1,639.8	0.1	483.7	1,375.7	268.4	(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 [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units			(1.2)
Share-based compensation expense			3.5
Net income	86.0			86.0
Dividends, 2021, $0.31, $0.62 per share; 2020, $0.30, $0.60 per share				(26.2)
Change in net unrealized investment gains (losses) on fixed maturity securities					(34.1)
Acquisition of shares	(1.5)
Ending balance at Jun. 30, 2021	1,816.6	0.1	490.7	1,494.4	321.0	(489.6)
Beginning balance at Mar. 31, 2021		0.1	489.2	1,460.8	232.4	(489.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense			1.5
Net income	46.7			46.7
Dividends, 2021, $0.31, $0.62 per share; 2020, $0.30, $0.60 per share				(13.1)
Change in net unrealized investment gains (losses) on fixed maturity securities					88.6
Ending balance at Jun. 30, 2021	$ 1,816.6	$ 0.1	$ 490.7	$ 1,494.4	$ 321.0	$ (489.6)