Investments - Net Investment Income (Parenthetical) (Details) - Deposit asset underlying 10% coinsurance agreement [Member] - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gains (Losses) recognized for change in fair value of deposit asset underlying the 10% coinsurance agreement	$ (0.8)	$ (6.4)
Percentage of coinsurance costs	10.00%	10.00%

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration