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Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Loss) - Components of Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2022
Sep. 30, 2022
Jun. 30, 2022
Mar. 31, 2022
Dec. 31, 2021
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]                      
Net income $ (16.7) $ 20.4 $ (4.2) $ 20.3 $ 46.9 $ 20.2 $ 54.2 $ 49.1 $ 45.0 $ 19.8 [1],[2] $ 170.4 [1],[2]
Effect of adopting ASU 2018-12                 0.0 0.0 [1] (426.6) [1]
Change in net unrealized investment gains (losses) on fixed maturity securities:                      
Net unrealized investment gains (losses) on securities arising during the period                 133.9 (1,042.6) (122.8)
Less: reclassification adjustment for net investment gains (losses) included in income before income tax                 (20.3) (29.1) (7.7)
Total, before tax                 154.2 (1,013.5) (115.1)
Income tax expense (benefit)                 32.9 (216.8) (24.8)
Total, net of tax                 121.3 (796.7) (90.3)
Change in net reserve remeasurements attributable to discount rates:                      
Total, before tax                 (47.2) 567.6 141.0
Income tax expense (benefit)                 (10.1) 121.7 30.2
Total, net of tax                 (37.1) 445.9 [1] 110.8 [1]
Change in net funded status of benefit plans:                      
Total, before tax                 1.5 1.8 1.2
Income tax expense (benefit)                 0.3 0.4 0.2
Total, net of tax                 1.2 1.4 [1] 1.0 [1]
Total comprehensive income (loss)                 $ 130.4 $ (329.6) [1] $ (234.7) [1]
[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