Schedule II Condensed Financial Information of Registrant - Condensed Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
ASSETS
Investments in subsidiaries	$ 1,100.6	$ 1,121.3
Total assets	15,266.6	14,487.8	$ 14,049.9
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	0.0	0.0
Long-term debt	593.4	547.0
Other liabilities	374.0	312.3
Total liabilities	13,783.9	13,200.3
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, 2025, 67,323,929; 2024, 67,032,164	0.1	0.1
Additional paid-in capital	538.8	525.2
Retained earnings	1,651.7	1,548.2
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains (losses) on fixed maturity securities	(245.5)	(357.4)
Net reserve remeasurements attributable to discount rates	93.4	110.9
Net funded status of benefit plans	(2.5)	(7.0)
Treasury stock, at cost, 2025, 26,664,472 shares; 2024, 26,167,246 shares	(553.3)	(532.5)
Total shareholders' equity	1,482.7	1,287.5	$ 1,175.3
Total liabilities and shareholders' equity	15,266.6	14,487.8
Parent company
ASSETS
Investments and cash	124.1	45.5
Investments in subsidiaries	1,976.9	1,794.9
Other Assets	22.1	3.3
Total assets	2,123.1	1,843.7
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	0.0	0.0
Long-term debt	593.4	547.0
Other liabilities	47.0	9.2
Total liabilities	640.4	556.2
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, 2025, 67,323,929; 2024, 67,032,164	0.1	0.1
Additional paid-in capital	538.8	525.2
Retained earnings	1,651.7	1,548.2
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains (losses) on fixed maturity securities	(245.5)	(357.4)
Net reserve remeasurements attributable to discount rates	93.4	110.9
Net funded status of benefit plans	(2.5)	(7.0)
Treasury stock, at cost, 2025, 26,664,472 shares; 2024, 26,167,246 shares	(553.3)	(532.5)
Total shareholders' equity	1,482.7	1,287.5
Total liabilities and shareholders' equity	$ 2,123.1	$ 1,843.7