CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Common stock, $0.001 par value	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income (loss), net of tax:	Treasury stock, at cost
Beginning balance at Dec. 31, 2020		$ 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, 2022, $0.32 per share; 2021, $0.31 per share				(39.4)
Change in net unrealized investment 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, 2022, $0.32 per share; 2021, $0.31 per share				(13.2)
Change in net unrealized investment 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)
Beginning balance at Dec. 31, 2021	1,807.4	0.1	495.3	1,524.9	280.5	(493.4)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common stock units and restricted stock units			(1.1)
Share-based compensation expense			6.2
Net income	15.9			15.9
Dividends, 2022, $0.32 per share; 2021, $0.31 per share				(40.4)
Change in net unrealized investment losses on fixed maturity securities					(687.4)
Acquisition of shares						(24.0)
Ending balance at Sep. 30, 2022	1,076.6	0.1	500.4	1,500.4	(406.9)	(517.4)
Beginning balance at Jun. 30, 2022		0.1	498.1	1,499.9	(230.6)	(507.4)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common stock units and restricted stock units			0.2
Share-based compensation expense			2.1
Net income	13.9			13.9
Dividends, 2022, $0.32 per share; 2021, $0.31 per share				(13.4)
Change in net unrealized investment losses on fixed maturity securities					(176.3)
Acquisition of shares						(10.0)
Ending balance at Sep. 30, 2022	$ 1,076.6	$ 0.1	$ 500.4	$ 1,500.4	$ (406.9)	$ (517.4)