Allowance for Credit Losses - Narrative (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019		Mar. 31, 2020	Jan. 01, 2020	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption	$ 5,984.5	$ 5,682.1		$ 5,386.2		$ 5,760.3	$ 5,471.7	$ 5,133.9
Total allowance for credit losses	51.0	20.3
Allowance for credit losses, period increase, net	7.7
Total increase in the allowance for credit losses	11.6
Allowance for credit loss, recovery	3.9
Allowance for credit losses with no impact on income	23.0
Allowance for credit losses writeoff	2.8
Reinsurance recoverables	9,581.9	$ 9,593.4			$ 9,593.4
Increase in the CECL allowance for reinsurance recoverable	4.0
Increase in the CECL allowance for premium and accounts receivables	$ 4.0
Percentage of residential mortgage-backed holdings exposure to sub-prime mortgage collateral	40.00%
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member	[1]
Allowance for credit losses	$ 4.1	$ 0.6			2.2
Retained earnings (accumulative deficit)	6,194.4	5,966.4
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption		(20.4)	[1]		(20.4)
Total allowance for credit losses					46.2
Allowance for credit losses with no impact on income	$ 25.9
Reinsurance recoverables					(22.5)
Retained earnings (accumulative deficit)					$ (1.3)
A- or Better Rating
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total percentage of recoverables subject to allowance	71.00%
BBB or BB Rating
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total percentage of recoverables subject to allowance	24.00%
Not rated
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total percentage of recoverables subject to allowance	5.00%
Commercial mortgage loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss, period increase	$ 1.9
Commercial Real Estate Portfolio Segment | Commercial mortgage loans | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Commercial mortgage loans on real estate	0.1	0.1
Commercial Real Estate Portfolio Segment | Commercial mortgage loans | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Commercial mortgage loans on real estate	12.2	$ 12.3
Underwriting, general and administrative expense
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, period increase, net	4.4
Net realized losses on investments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, period increase, net	3.3
Iké
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, period increase, net	$ 1.4

[1] Amount relates to the adoption of a new accounting standard for accounting for expected credit losses for assets held at amortized cost, which established allowances for such expected credit losses as of January 1, 2020. Refer to Notes 3 and 6 for additional information.