Schedule II Condensed Financial Information of Registrant - Condensed Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
ASSETS
Investments in subsidiaries	$ 1,138.8	$ 983.7	[1]
Total assets	14,049.9	13,306.1	[1]	$ 14,460.2
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	0.0	249.0	[1]
Long-term debt	546.0	249.0	[1]
Other liabilities	287.1	299.5	[1]
Total liabilities	12,874.6	12,207.8	[1]
Preferred stock, $0.001 par value, authorized 1,000,000 shares; none issued	0.0	0.0	[1]
Common stock, $0.001 par value, authorized 75,000,000 shares; issued, 2023, 66,747,821; 2022, 66,618,465	0.1	0.1	[1]
Additional paid-in capital	510.9	502.6	[1]
Retained earnings	1,502.2	1,512.4	[1]
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains (losses) on fixed maturity securities	(328.3)	(449.6)	[1]
Net reserve remeasurements attributable to discount rates	21.9	59.0	[1]
Net funded status of benefit plans	(7.6)	(8.8)	[1]
Treasury stock, at cost, 2023, 25,911,087 shares; 2022, 25,714,153 shares	(523.9)	(517.4)	[1]
Total shareholders' equity	1,175.3	1,098.3	[2]	$ 1,499.0	[2]
Total liabilities and shareholders' equity	$ 14,049.9	$ 13,306.1	[1]
Common Stock, Shares, Issued	66,747,821	66,618,465
Parent company
ASSETS
Investments and cash	$ 3.4	$ 2.1
Investments in subsidiaries	1,716.6	1,597.1
Other Assets	17.1	7.1
Total assets	1,737.1	1,606.3
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	0.0	249.0
Long-term debt	546.0	249.0
Other liabilities	15.8	10.0
Total liabilities	561.8	508.0
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, 2023, 66,747,821; 2022, 66,618,465	0.1	0.1
Additional paid-in capital	510.9	502.6
Retained earnings	1,502.2	1,512.4
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains (losses) on fixed maturity securities	(328.3)	(449.6)
Net reserve remeasurements attributable to discount rates	21.9	59.0
Net funded status of benefit plans	(7.6)	(8.8)
Treasury stock, at cost, 2023, 25,911,087 shares; 2022, 25,714,153 shares	(523.9)	(517.4)
Total shareholders' equity	1,175.3	1,098.3
Total liabilities and shareholders' equity	$ 1,737.1	$ 1,606.3
Common Stock, Shares, Issued	66,747,821	66,618,465

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements

[2] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements