Schedule 2 - Parent Company Financial Statements - Balance Sheets (Additional Information) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Jun. 07, 2019
Amortized cost of fixed maturities	$ 6,692.7	$ 6,372.7
Debt	$ 1,247.8	$ 820.2
Senior Notes \| Senior Notes Payable, 4.35% due 2025 (Fair Value: 2020 – $499.5; 2019 – $478.6)
Stated interest rate, percentage	4.35%	4.35%
Subordinated Debt \| Subordinated Debentures due 2054 (Fair Value: 2018 – $151.1)
Stated interest rate, percentage			7.375%
Parent Company
Amortized cost of fixed maturities	$ 91.8
Parent Company \| Secured Debt \| Term Loan due July 5, 2023 (Fair Value: 2020 – $50.0; 2019 – $50.0)
Debt	50.0	$ 50.0
Parent Company \| Senior Notes \| Senior Notes Payable, 4.35% due 2025 (Fair Value: 2020 – $499.5; 2019 – $478.6)
Debt	$ 499.5	$ 478.6

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-SubTopic 10
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