Schedule II Condensed Financial Information of Registrant - Condensed Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
ASSETS
Investments in subsidiaries	$ 983.7	$ 712.8
Other assets	328.7	288.1
Total assets	13,446.8	14,383.9	$ 13,471.8
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	249.0	249.0
Long-term debt	249.0	253.6
Other liabilities	297.0	428.2
Total liabilities	12,358.6	12,576.5
Preferred stock, $0.001 par value, authorized 1,000,000 shares; none issued	0.0	0.0
Common stock, $0.001 par value, authorized 75,000,000 shares; issued, 2022, 66,618,465; 2021, 75,000,000	0.1	0.1
Additional paid-in capital	502.6	495.3
Retained earnings	1,468.6	1,524.9
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains (losses) on fixed maturity securities	(356.9)	290.7
Net funded status of benefit plans	(8.8)	(10.2)
Treasury stock, at cost, 2022, 25,714,153 shares; 2021, 24,043,337 shares	(517.4)	(493.4)
Total shareholders' equity	1,088.2	1,807.4	$ 1,790.1
Total liabilities and shareholders' equity	13,446.8	14,383.9
Parent company
ASSETS
Investments and cash	2.1	115.3
Investments in subsidiaries	1,587.0	2,191.3
Other assets	7.1	11.3
Total assets	1,596.2	2,317.9
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	249.0	249.0
Long-term debt	249.0	248.6
Other liabilities	10.0	12.9
Total liabilities	508.0	510.5
Preferred stock, $0.001 par value, authorized 1,000,000 shares; none issued	0.0	0.0
Common stock, $0.001 par value, authorized 75,000,000 shares; issued, 2022, 66,618,465; 2021, 75,000,000	0.1	0.1
Additional paid-in capital	502.6	495.3
Retained earnings	1,468.6	1,524.9
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains (losses) on fixed maturity securities	(356.9)	290.7
Net funded status of benefit plans	(8.8)	(10.2)
Treasury stock, at cost, 2022, 25,714,153 shares; 2021, 24,043,337 shares	(517.4)	(493.4)
Total shareholders' equity	1,088.2	1,807.4
Total liabilities and shareholders' equity	$ 1,596.2	$ 2,317.9