Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
AOCI Attributable to Parent, Net of Tax
Beginning balance	$ 1,790.1	$ 1,567.3
Other comprehensive income (loss) before reclassifications	(81.3)	144.3	$ 259.3
Amounts reclassified from AOCI	6.7	(8.8)	(124.4)
Net current period other comprehensive income (loss)	(74.6)	135.5	134.9
Ending balance	1,807.4	1,790.1	1,567.3
Reclassification adjustment for net investment gains (losses) included in income before income tax	(8.5)	11.2	157.4
Other tax expense (benefit)	(1.8)	2.4	33.0
Total
AOCI Attributable to Parent, Net of Tax
Beginning balance	355.1	219.6	84.7
Ending balance	280.5	355.1	219.6
Net unrealized investment gains (losses) on securities
AOCI Attributable to Parent, Net of Tax
Beginning balance	366.3	230.4	96.9
Other comprehensive income (loss) before reclassifications	(82.3)	144.7	257.9
Amounts reclassified from AOCI	6.7	(8.8)	(124.4)
Net current period other comprehensive income (loss)	(75.6)	135.9	133.5
Ending balance	290.7	366.3	230.4
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax
Beginning balance	(11.2)	(10.8)	(12.2)
Other comprehensive income (loss) before reclassifications	1.0	(0.4)	1.4
Amounts reclassified from AOCI	0.0	0.0	0.0
Net current period other comprehensive income (loss)	1.0	(0.4)	1.4
Ending balance	$ (10.2)	$ (11.2)	$ (10.8)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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