Investments - Reconciliation of Net Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance			$ 1,790.1
Ending balance	$ 1,796.4	$ 1,715.8	1,796.4	$ 1,715.8
Net unrealized investment gains (losses) on securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	332.2	279.1	366.3	230.4
Ending balance	306.9	328.2	306.9	328.2
Net unrealized investment gains (losses) on securities \| Fixed maturity securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	399.4	329.9	439.8	264.4
Change in net unrealized investment gains (losses) on fixed maturity securities	(34.9)	56.9	(77.0)	128.1
Reclassification of net investment (gains) losses on fixed maturity securities to net income	3.9	5.2	5.6	(0.5)
Ending balance	$ 368.4	$ 392.0	$ 368.4	$ 392.0

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.

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Reference 2: http://www.xbrl.org/2003/role/exampleRef
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Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
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+ Details