Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Financial Assets
Equity securities	$ 99.6	$ 147.2
Carrying Value
Financial Assets
Equity securities	99.6	147.2
Short-term investments	109.4	157.8
Other investments	38.6	43.6
Investments, Fair Value Disclosure, Total	5,432.6	6,587.9
Separate account (variable annuity) assets	2,792.3	3,441.0
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	1.2	2.1
Other policyholder funds, embedded derivatives	91.0	106.6
Carrying Value | Recurring
Financial Assets
Total fixed maturity securities	5,185.0	6,239.3
Carrying Value | Mortgage-backed securities
Financial Assets
Total fixed maturity securities		662.5
Carrying Value | Mortgage-backed securities | Recurring
Financial Assets
Total fixed maturity securities	570.4
Carrying Value | Other, including U.S. Treasury securities
Financial Assets
Total fixed maturity securities		365.9
Carrying Value | Other, including U.S. Treasury securities | Recurring
Financial Assets
Total fixed maturity securities	342.6
Carrying Value | Municipal bonds
Financial Assets
Total fixed maturity securities		1,703.4
Carrying Value | Municipal bonds | Recurring
Financial Assets
Total fixed maturity securities	1,269.7
Carrying Value | Foreign government bonds
Financial Assets
Total fixed maturity securities		43.6
Carrying Value | Foreign government bonds | Recurring
Financial Assets
Total fixed maturity securities	33.6
Carrying Value | Corporate bonds
Financial Assets
Total fixed maturity securities		2,388.7
Carrying Value | Corporate bonds | Recurring
Financial Assets
Total fixed maturity securities	1,901.7
Carrying Value | Other asset-backed securities
Financial Assets
Total fixed maturity securities		1,075.2
Carrying Value | Other asset-backed securities | Recurring
Financial Assets
Total fixed maturity securities	1,067.0
Fair Value
Financial Assets
Equity securities	99.6	147.2
Short-term investments	109.4	157.8
Other investments	38.6	43.6
Investments, Fair Value Disclosure, Total	5,432.6	6,587.9
Separate account (variable annuity) assets	2,792.3	3,441.0
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	1.2	2.1
Other policyholder funds, embedded derivatives	91.0	106.6
Fair Value | Recurring
Financial Assets
Total fixed maturity securities	5,185.0	6,239.3
Fair Value | Level 1
Financial Assets
Equity securities	23.3	35.2
Short-term investments	109.4	157.8
Other investments	0.0	0.0
Investments, Fair Value Disclosure, Total	169.5	225.6
Separate account (variable annuity) assets	2,792.3	3,441.0
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	0.0	0.0
Other policyholder funds, embedded derivatives	0.0	0.0
Fair Value | Level 1 | Recurring
Financial Assets
Total fixed maturity securities	36.8	32.6
Fair Value | Level 2
Financial Assets
Equity securities	74.3	110.6
Short-term investments	0.0	0.0
Other investments	38.6	43.6
Investments, Fair Value Disclosure, Total	4,837.8	5,990.9
Separate account (variable annuity) assets	0.0	0.0
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	1.2	2.1
Other policyholder funds, embedded derivatives	0.0	0.0
Fair Value | Level 2 | Recurring
Financial Assets
Total fixed maturity securities	4,724.9	5,836.7
Fair Value | Level 3
Financial Assets
Equity securities	2.0	1.4
Short-term investments	0.0	0.0
Other investments	0.0	0.0
Investments, Fair Value Disclosure, Total	425.3	371.4
Separate account (variable annuity) assets	0.0	0.0
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	0.0	0.0
Other policyholder funds, embedded derivatives		106.6
Fair Value | Level 3 | Recurring
Financial Assets
Total fixed maturity securities	423.3	370.0
Fair Value | Mortgage-backed securities
Financial Assets
Total fixed maturity securities		662.5
Fair Value | Mortgage-backed securities | Recurring
Financial Assets
Total fixed maturity securities	570.4
Fair Value | Mortgage-backed securities | Level 1
Financial Assets
Total fixed maturity securities		0.0
Fair Value | Mortgage-backed securities | Level 1 | Recurring
Financial Assets
Total fixed maturity securities	0.0
Fair Value | Mortgage-backed securities | Level 2
Financial Assets
Total fixed maturity securities		662.5
Fair Value | Mortgage-backed securities | Level 2 | Recurring
Financial Assets
Total fixed maturity securities	567.8
Fair Value | Mortgage-backed securities | Level 3
Financial Assets
Total fixed maturity securities		0.0
Fair Value | Mortgage-backed securities | Level 3 | Recurring
Financial Assets
Total fixed maturity securities	2.6
Fair Value | Other, including U.S. Treasury securities
Financial Assets
Total fixed maturity securities		365.9
Fair Value | Other, including U.S. Treasury securities | Recurring
Financial Assets
Total fixed maturity securities	342.6
Fair Value | Other, including U.S. Treasury securities | Level 1
Financial Assets
Total fixed maturity securities		17.7
Fair Value | Other, including U.S. Treasury securities | Level 1 | Recurring
Financial Assets
Total fixed maturity securities	24.6
Fair Value | Other, including U.S. Treasury securities | Level 2
Financial Assets
Total fixed maturity securities		348.2
Fair Value | Other, including U.S. Treasury securities | Level 2 | Recurring
Financial Assets
Total fixed maturity securities	318.0
Fair Value | Other, including U.S. Treasury securities | Level 3
Financial Assets
Total fixed maturity securities		0.0
Fair Value | Other, including U.S. Treasury securities | Level 3 | Recurring
Financial Assets
Total fixed maturity securities	0.0
Fair Value | Municipal bonds
Financial Assets
Total fixed maturity securities		1,703.4
Fair Value | Municipal bonds | Recurring
Financial Assets
Total fixed maturity securities	1,269.7
Fair Value | Municipal bonds | Level 1
Financial Assets
Total fixed maturity securities		0.0
Fair Value | Municipal bonds | Level 1 | Recurring
Financial Assets
Total fixed maturity securities	0.0
Fair Value | Municipal bonds | Level 2
Financial Assets
Total fixed maturity securities		1,642.6
Fair Value | Municipal bonds | Level 2 | Recurring
Financial Assets
Total fixed maturity securities	1,215.3
Fair Value | Municipal bonds | Level 3
Financial Assets
Total fixed maturity securities	54.4	60.8
Fair Value | Municipal bonds | Level 3 | Recurring
Financial Assets
Total fixed maturity securities	54.4
Fair Value | Foreign government bonds
Financial Assets
Total fixed maturity securities		43.6
Fair Value | Foreign government bonds | Recurring
Financial Assets
Total fixed maturity securities	33.6
Fair Value | Foreign government bonds | Level 1
Financial Assets
Total fixed maturity securities		0.0
Fair Value | Foreign government bonds | Level 1 | Recurring
Financial Assets
Total fixed maturity securities	0.0
Fair Value | Foreign government bonds | Level 2
Financial Assets
Total fixed maturity securities		43.6
Fair Value | Foreign government bonds | Level 2 | Recurring
Financial Assets
Total fixed maturity securities	33.6
Fair Value | Foreign government bonds | Level 3
Financial Assets
Total fixed maturity securities		0.0
Fair Value | Foreign government bonds | Level 3 | Recurring
Financial Assets
Total fixed maturity securities	0.0
Fair Value | Corporate bonds
Financial Assets
Total fixed maturity securities		2,388.7
Fair Value | Corporate bonds | Recurring
Financial Assets
Total fixed maturity securities	1,901.7
Fair Value | Corporate bonds | Level 1
Financial Assets
Total fixed maturity securities		14.9
Fair Value | Corporate bonds | Level 1 | Recurring
Financial Assets
Total fixed maturity securities	12.2
Fair Value | Corporate bonds | Level 2
Financial Assets
Total fixed maturity securities		2,163.5
Fair Value | Corporate bonds | Level 2 | Recurring
Financial Assets
Total fixed maturity securities	1,628.2
Fair Value | Corporate bonds | Level 3
Financial Assets
Total fixed maturity securities	261.3	210.3
Fair Value | Corporate bonds | Level 3 | Recurring
Financial Assets
Total fixed maturity securities	261.3
Fair Value | Other asset-backed securities
Financial Assets
Total fixed maturity securities		1,075.2
Fair Value | Other asset-backed securities | Recurring
Financial Assets
Total fixed maturity securities	1,067.0
Fair Value | Other asset-backed securities | Level 1
Financial Assets
Total fixed maturity securities		0.0
Fair Value | Other asset-backed securities | Level 1 | Recurring
Financial Assets
Total fixed maturity securities	0.0
Fair Value | Other asset-backed securities | Level 2
Financial Assets
Total fixed maturity securities		976.3
Fair Value | Other asset-backed securities | Level 2 | Recurring
Financial Assets
Total fixed maturity securities	962.0
Fair Value | Other asset-backed securities | Level 3
Financial Assets
Total fixed maturity securities		$ 98.9
Fair Value | Other asset-backed securities | Level 3 | Recurring
Financial Assets
Total fixed maturity securities	$ 105.0