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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock, par value (in usd per share)	$ 0.001
Beginning balance at Dec. 31, 2019		$ 0.1	$ 481.0	$ 1,352.5	$ 219.7	$ (486.0)	$ (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			1.1
Net income	$ 18.5			18.5
Dividends, 2021, $0.31 per share; 2020, $0.30 per share				(12.7)
Change in net unrealized investment gains (losses) on fixed maturity securities					(93.8)
Acquisition of shares	(2.1)
Ending balance at Mar. 31, 2020	$ 1,477.6	0.1	481.9	1,357.8	125.9	(488.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock, par value (in usd per share)	$ 0.001
Common stock, par value (in usd per share)	$ 0.001
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			2.0
Net income	39.3			39.3
Dividends, 2021, $0.31 per share; 2020, $0.30 per share				(13.1)
Change in net unrealized investment gains (losses) on fixed maturity securities					(122.7)
Acquisition of shares	(1.5)
Ending balance at Mar. 31, 2021	$ 1,692.9	$ 0.1	$ 489.2	$ 1,460.8	$ 232.4	$ (489.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock, par value (in usd per share)	$ 0.001