Investments - Information About Credit Quality and Vintage Year of Commercial Mortgage Loans (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 11,731.4	$ 10,727.5
Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	1,486.6	1,412.7
Year two	1,378.5	1,754.8
Year three	1,717.3	1,885.6
Year four	1,760.1	1,805.5
Year five	1,489.1	1,414.8
Prior	2,953.6	2,001.5
Revolving Loans	4.1	4.0
Total	$ 10,789.3	$ 10,278.9
% of Total	100.00%	100.00%
Greater than 1.20x \| Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	$ 932.5	$ 1,078.4
Year two	903.0	1,601.7
Year three	1,601.9	1,738.0
Year four	1,395.9	1,794.4
Year five	1,355.4	1,408.8
Prior	2,689.6	1,880.6
Revolving Loans	4.1	4.0
Total	$ 8,882.4	$ 9,505.9
% of Total	82.00%	93.00%
1.00x - 1.20x \| Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	$ 554.1	$ 334.3
Year two	346.7	137.9
Year three	96.9	89.7
Year four	90.9	11.1
Year five	11.0	0.0
Prior	69.1	88.8
Revolving Loans	0.0	0.0
Total	$ 1,168.7	$ 661.8
% of Total	11.00%	6.00%
Less than 1.00x \| Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	$ 0.0	$ 0.0
Year two	128.8	15.2
Year three	18.5	57.9
Year four	273.3	0.0
Year five	122.7	6.0
Prior	194.9	32.1
Revolving Loans	0.0	0.0
Total	$ 738.2	$ 111.2
% of Total	7.00%	1.00%
Less than 70% \| Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	$ 1,167.0	$ 1,346.5
Year two	1,324.8	1,315.0
Year three	1,347.2	1,752.8
Year four	1,635.7	1,678.7
Year five	1,412.7	1,320.5
Prior	2,764.1	1,846.3
Revolving Loans	4.1	4.0
Total	$ 9,655.6	$ 9,263.8
% of Total	89.00%	90.00%
70% - 80% \| Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	$ 319.6	$ 66.2
Year two	53.7	348.1
Year three	312.5	127.9
Year four	119.6	80.0
Year five	49.0	94.3
Prior	179.9	128.5
Revolving Loans	0.0	0.0
Total	$ 1,034.3	$ 845.0
% of Total	10.00%	8.00%
80% - 100% \| Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	$ 0.0	$ 0.0
Year two	0.0	91.7
Year three	42.9	4.9
Year four	4.8	46.8
Year five	27.4	0.0
Prior	9.6	26.7
Revolving Loans	0.0	0.0
Total	$ 84.7	$ 170.1
% of Total	1.00%	2.00%
Greater than 100% \| Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	$ 0.0	$ 0.0
Year two	0.0	0.0
Year three	14.7	0.0
Year four	0.0	0.0
Year five	0.0	0.0
Prior	0.0	0.0
Revolving Loans	0.0	0.0
Total	$ 14.7	$ 0.0
% of Total	0.00%	0.00%

Percentage, before allowance for credit loss, of financing receivable to total financing receivable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
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Amount excluding accrued interest, before allowance for credit loss, of financing receivable.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
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Amount of financing receivable originated more than five years prior to current fiscal year.

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Amount of financing receivable originated four years prior to current fiscal year.

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Amount of financing receivable originated three years prior to current fiscal year.

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