Schedule II Condensed Financial Information of Registrant - Condensed Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
ASSETS
Investments in subsidiaries	$ 1,121.3	$ 1,138.8
Total assets	14,487.8	14,049.9	$ 13,306.1
LIABILITIES AND SHAREHOLDERS' EQUITY
Long-term debt	547.0	546.0
Other liabilities	312.3	287.1
Total liabilities	13,200.3	12,874.6
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, 2024, 67,032,164; 2023, 66,747,821	0.1	0.1
Additional paid-in capital	525.2	510.9
Retained earnings	1,548.2	1,502.2
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains (losses) on fixed maturity securities	(357.4)	(328.3)
Net reserve remeasurements attributable to discount rates	110.9	21.9
Net funded status of benefit plans	(7.0)	(7.6)
Treasury stock, at cost, 2024, 26,167,246 shares; 2023, 25,911,087 shares	(532.5)	(523.9)
Total shareholders' equity	1,287.5	1,175.3	$ 1,098.3	[1]
Total liabilities and shareholders' equity	$ 14,487.8	$ 14,049.9
Common Stock, Shares, Issued	67,032,164	66,747,821
Parent company
ASSETS
Investments and cash	$ 45.5	$ 3.4
Investments in subsidiaries	1,794.9	1,716.6
Other Assets	3.3	17.1
Total assets	1,843.7	1,737.1
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	0.0	0.0
Long-term debt	547.0	546.0
Other liabilities	9.2	15.8
Total liabilities	556.2	561.8
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, 2024, 67,032,164; 2023, 66,747,821	0.1	0.1
Additional paid-in capital	525.2	510.9
Retained earnings	1,548.2	1,502.2
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains (losses) on fixed maturity securities	(357.4)	(328.3)
Net reserve remeasurements attributable to discount rates	110.9	21.9
Net funded status of benefit plans	(7.0)	(7.6)
Treasury stock, at cost, 2024, 26,167,246 shares; 2023, 25,911,087 shares	(532.5)	(523.9)
Total shareholders' equity	1,287.5	1,175.3
Total liabilities and shareholders' equity	$ 1,843.7	$ 1,737.1
Common Stock, Shares, Issued	67,032,164	66,747,821

[1] Recast for the adoption of ASU 2018-12.