Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) $ in Millions	12 Months Ended
Dec. 31, 2025 USD ($) investment	Dec. 31, 2024 USD ($) investment
Financial Assets
Total fixed maturity securities	$ 5,714.6	$ 5,387.9
Equity securities	41.9	66.5
Limited partnership interests	$ 1,100.6	$ 1,121.3
Number of limited partnership investments sold \| investment	2
Number of limited partnership investments \| investment		2
Carrying Amount
Financial Assets
Total fixed maturity securities	$ 5,714.6	$ 5,387.9
Equity securities	41.9	66.5
Limited partnership interests		28.9
Short-term investments	210.5	101.1
Other investments	21.6	18.5
Totals	5,988.6	5,602.9
Separate account (variable annuity) assets	4,157.4	3,708.8
Financial Liabilities	78.9	79.6
Carrying Amount \| Mortgage-backed securities
Financial Assets
Total fixed maturity securities	689.0	755.8
Carrying Amount \| Other, including U.S. Treasury securities
Financial Assets
Total fixed maturity securities	325.8	357.6
Carrying Amount \| Municipal Bonds
Financial Assets
Total fixed maturity securities	1,177.1	1,150.8
Carrying Amount \| Foreign government bonds
Financial Assets
Total fixed maturity securities	10.0	13.1
Carrying Amount \| Corporate Bonds
Financial Assets
Total fixed maturity securities	1,910.3	1,782.4
Carrying Amount \| Other asset-backed securities
Financial Assets
Total fixed maturity securities	1,602.4	1,328.2
Fair Value
Financial Assets
Total fixed maturity securities	5,714.6	5,387.9
Equity securities	41.9	66.5
Limited partnership interests		28.9
Short-term investments	210.5	101.1
Other investments	21.6	18.5
Totals	5,988.6	5,602.9
Separate account (variable annuity) assets	4,157.4	3,708.8
Financial Liabilities	78.9	79.6
Fair Value \| Level 1
Financial Assets
Total fixed maturity securities	45.7	32.4
Equity securities	1.3	1.4
Limited partnership interests		0.0
Short-term investments	210.5	101.1
Other investments	0.0	0.0
Totals	257.5	134.9
Separate account (variable annuity) assets	4,157.4	3,708.8
Financial Liabilities	0.0	0.0
Fair Value \| Level 2
Financial Assets
Total fixed maturity securities	5,203.6	4,855.6
Equity securities	36.8	60.9
Limited partnership interests		0.0
Short-term investments	0.0	0.0
Other investments	21.6	18.5
Totals	5,262.0	4,935.0
Separate account (variable annuity) assets		0.0
Financial Liabilities	5.8	4.1
Fair Value \| Level 3
Financial Assets
Total fixed maturity securities	465.3	499.9
Equity securities	3.8	4.2
Limited partnership interests		28.9
Short-term investments	0.0	0.0
Other investments	0.0	0.0
Totals	469.1	533.0
Separate account (variable annuity) assets		0.0
Financial Liabilities	73.1	75.5
Fair Value \| Mortgage-backed securities
Financial Assets
Total fixed maturity securities	689.0	755.8
Fair Value \| Mortgage-backed securities \| Level 1
Financial Assets
Total fixed maturity securities	0.0	0.0
Fair Value \| Mortgage-backed securities \| Level 2
Financial Assets
Total fixed maturity securities	689.0	755.8
Fair Value \| Mortgage-backed securities \| Level 3
Financial Assets
Total fixed maturity securities	0.0	0.0
Fair Value \| Other, including U.S. Treasury securities
Financial Assets
Total fixed maturity securities	325.8	357.6
Fair Value \| Other, including U.S. Treasury securities \| Level 1
Financial Assets
Total fixed maturity securities	40.8	24.7
Fair Value \| Other, including U.S. Treasury securities \| Level 2
Financial Assets
Total fixed maturity securities	285.0	332.9
Fair Value \| Other, including U.S. Treasury securities \| Level 3
Financial Assets
Total fixed maturity securities	0.0	0.0
Fair Value \| Municipal Bonds
Financial Assets
Total fixed maturity securities	1,177.1	1,150.8
Fair Value \| Municipal Bonds \| Level 1
Financial Assets
Total fixed maturity securities	0.0	0.0
Fair Value \| Municipal Bonds \| Level 2
Financial Assets
Total fixed maturity securities	1,098.7	1,075.9
Fair Value \| Municipal Bonds \| Level 3
Financial Assets
Total fixed maturity securities	78.4	74.9
Fair Value \| Foreign government bonds
Financial Assets
Total fixed maturity securities	10.0	13.1
Fair Value \| Foreign government bonds \| Level 1
Financial Assets
Total fixed maturity securities	0.0	0.0
Fair Value \| Foreign government bonds \| Level 2
Financial Assets
Total fixed maturity securities	10.0	13.1
Fair Value \| Foreign government bonds \| Level 3
Financial Assets
Total fixed maturity securities	0.0	0.0
Fair Value \| Corporate Bonds
Financial Assets
Total fixed maturity securities	1,910.3	1,782.4
Fair Value \| Corporate Bonds \| Level 1
Financial Assets
Total fixed maturity securities	4.9	7.7
Fair Value \| Corporate Bonds \| Level 2
Financial Assets
Total fixed maturity securities	1,562.8	1,423.4
Fair Value \| Corporate Bonds \| Level 3
Financial Assets
Total fixed maturity securities	342.6	351.3
Fair Value \| Other asset-backed securities
Financial Assets
Total fixed maturity securities	1,602.4	1,328.2
Fair Value \| Other asset-backed securities \| Level 1
Financial Assets
Total fixed maturity securities	0.0	0.0
Fair Value \| Other asset-backed securities \| Level 2
Financial Assets
Total fixed maturity securities	1,558.1	1,254.5
Fair Value \| Other asset-backed securities \| Level 3
Financial Assets
Total fixed maturity securities	$ 44.3	$ 73.7

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