Schedule II. Condensed Financial Information of Registrant (Condensed Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments
Fixed maturity securities at fair value (amortized cost $9.1 at December 31, 2021 and $9.6 at December 31, 2020)	$ 2,342.7	$ 2,479.2
Equity securities at fair value (cost $25.2 at December 31, 2021 and $0.4 at December 31, 2020)	338.8	208.5
Total investments	2,736.0	2,757.2
Cash and cash equivalents	75.1	160.4
Accrued investment income	14.5	15.3
Other assets	48.7	60.1
Total assets	3,783.2	3,922.6
Liabilities and stockholders' equity
Accounts payable and accrued expenses	24.1	22.9
Deferred income taxes, net	7.7	15.5
Other liabilities	57.6	70.5
Total liabilities	2,570.1	2,709.8
Stockholders' equity:
Common stock, $0.01 par value; 150,000,000 shares authorized; 57,690,254 and 57,413,806 shares issued and 27,741,400 and 28,564,798 shares outstanding at December 31, 2021 and 2020, respectively	0.6	0.6
Preferred stock, $0.01 par value; 25,000,000 shares authorized; none issued	0.0	0.0
Additional paid-in capital	410.7	404.3
Retained earnings	1,338.5	1,247.9
Accumulated other comprehensive income, net of tax	60.6	115.1
Treasury stock, at cost (29,948,854 shares at December 31, 2021 and 28,849,008 shares at December 31, 2020)	(597.3)	(555.1)
Total stockholders’ equity	1,213.1	1,212.8
Total liabilities and stockholders’ equity	3,783.2	3,922.6
Parent Company [Member]
Investments
Investment in subsidiaries	1,172.7	1,186.1
Fixed maturity securities at fair value (amortized cost $9.1 at December 31, 2021 and $9.6 at December 31, 2020)	10.1	11.0
Equity securities at fair value (cost $25.2 at December 31, 2021 and $0.4 at December 31, 2020)	25.0	0.4
Total investments	1,207.8	1,197.5
Cash and cash equivalents	4.8	11.0	$ 9.9	$ 41.3
Accrued investment income	0.2	0.2
Federal income taxes receivable	0.1	0.0
Federal income taxes receivable	1.5	0.8
Deferred Income Tax Assets, Net	2.1	1.7
Other assets	0.9	6.4
Total assets	1,217.4	1,217.6
Liabilities and stockholders' equity
Accounts payable and accrued expenses	4.3	4.7
Other liabilities	0.0	0.1
Total liabilities	4.3	4.8
Stockholders' equity:
Common stock, $0.01 par value; 150,000,000 shares authorized; 57,690,254 and 57,413,806 shares issued and 27,741,400 and 28,564,798 shares outstanding at December 31, 2021 and 2020, respectively	0.6	0.6
Preferred stock, $0.01 par value; 25,000,000 shares authorized; none issued	0.0	0.0
Additional paid-in capital	410.7	404.3
Retained earnings	1,338.5	1,247.9
Accumulated other comprehensive income, net of tax	60.6	115.1
Treasury stock, at cost (29,948,854 shares at December 31, 2021 and 28,849,008 shares at December 31, 2020)	(597.3)	(555.1)
Total stockholders’ equity	1,213.1	1,212.8
Total liabilities and stockholders’ equity	1,217.4	1,217.6
Short-term Investments [Member]
Investments
Fixed maturity securities at fair value (amortized cost $9.1 at December 31, 2021 and $9.6 at December 31, 2020)	$ 10.5	$ 26.6