Schedule 2 - Parent Company Financial Statements - Balance Sheets (Additional Information) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jun. 07, 2019	Dec. 31, 2018
Amortized cost of fixed maturities	$ 6,372.7		$ 6,284.5
Debt	$ 820.2		911.2
Senior Notes \| Senior Notes Payable, 4.35% due 2025 (Fair Value: 2019 – $478.6; 2018 – $444.2)
Stated interest rate, percentage	4.35%
Subordinated Debt \| Subordinated Debentures due 2054 (Fair Value: 2018 – $151.1)
Stated interest rate, percentage	7.375%	7.375%
Parent Company
Amortized cost of fixed maturities	$ 0.0		12.6
Parent Company \| Secured Debt \| Term Loan due June 29, 2020 (Fair Value: 2018 – $35.0)
Debt	0.0		35.0
Parent Company \| Secured Debt \| Term Loan due July 5, 2023 (Fair Value: 2019 – $50.0)
Debt	50.0		0.0
Parent Company \| Senior Notes \| Senior Notes Payable, 4.35% due 2025 (Fair Value: 2019 – $478.6; 2018 – $444.2)
Debt	$ 478.6		$ 444.2
Stated interest rate, percentage	4.35%		4.35%
Parent Company \| Subordinated Debt \| Subordinated Debentures due 2054 (Fair Value: 2018 – $151.1)
Debt	$ 0.0		$ 151.1
Stated interest rate, percentage	7.375%		7.375%

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
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-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=118939692&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=118942338&loc=SL82898722-210454

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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