Investments - Available-for-sale Securities with Cost Basis in Excess of Fair Value - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Available-for-sale fixed-maturities with cost basis in excess of fair values, cost basis	$ 267.5	$ 267.5	$ 234.8
Available-for-sale credit losses	0.5	4.2
Credit losses adjustment to amortized cost	$ 0.1	$ 3.8

Amount of increase in allowance for credit loss on investment in debt security measured at amortized cost (held-to-maturity), from recovery.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

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