Investments - Net Unrealized Investment Gains and Losses on Fixed Maturities and Equity Securities (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
Ending balance	1,807.4	1,790.1	$ 1,567.3
Net unrealized investment gains (losses) on securities
AOCI Attributable to Parent, Net of Tax
Beginning balance	366.3	230.4	96.9
Ending balance	290.7	366.3	230.4
Net unrealized investment gains (losses) on securities \| Fixed maturity securities
AOCI Attributable to Parent, Net of Tax
Beginning balance	439.8	264.4	111.7
Change in net unrealized investment (losses) gains on fixed maturity securities	(97.6)	184.2	277.1
Reclassification of net investment losses (gains) on fixed maturity securities to net income	6.7	(8.8)	(124.4)
Ending balance	$ 348.9	$ 439.8	$ 264.4

Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
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-Section 55
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details