Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,807.4	$ 1,790.1
Other comprehensive income (loss) before reclassifications	(694.9)	(81.3)	$ 144.3
Amounts reclassified from AOCI	48.7	6.7	(8.8)
Net current period other comprehensive income (loss)	(646.2)	(74.6)	135.5
Ending balance	1,088.2	1,807.4	1,790.1
Reclassification adjustment for net investment gains (losses) included in income before income tax	(61.6)	(8.5)	11.2
Other tax expense (benefit)	(12.9)	(1.8)	2.4
Net unrealized investment gains (losses) on securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	290.7	366.3	230.4
Other comprehensive income (loss) before reclassifications	(696.3)	(82.3)	144.7
Amounts reclassified from AOCI	48.7	6.7	(8.8)
Net current period other comprehensive income (loss)	(647.6)	(75.6)	135.9
Ending balance	(356.9)	290.7	366.3
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(10.2)	(11.2)	(10.8)
Other comprehensive income (loss) before reclassifications	1.4	1.0	(0.4)
Amounts reclassified from AOCI	0.0	0.0	0.0
Net current period other comprehensive income (loss)	1.4	1.0	(0.4)
Ending balance	(8.8)	(10.2)	(11.2)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	280.5	355.1	219.6
Ending balance	$ (365.7)	$ 280.5	$ 355.1