Fair Value Summary of Quantitative Information about Level 3 Fair Value Measurements (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 47,400.6	$ 40,342.3	$ 47,530.3
Fair Value, Inputs, Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	54.8	28.6	54.8
Fair Value, Inputs, Level 3 \| Internal Pricing \| Nonredeemable preferred stocks \| Financials
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 25.0	$ 13.1	$ 25.0
Fair Value Inputs, Original Issue Price	$ 3.7		$ 3.7
Fair Value Inputs, Guideline Public Company Method		$ 4.4
Fair Value, Inputs, Level 3 \| Internal Pricing \| Nonredeemable preferred stocks \| Industrials
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value		$ 7.1
Fair Value Inputs, Guideline Public Company Method		$ 5.6
Fair Value, Inputs, Level 3 \| Unadjusted Purchase Price \| Nonredeemable preferred stocks \| Financials
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 10.0		$ 10.0
Fair Value Inputs, Original Issue Price	$ 16.9		$ 16.9
Fair Value, Inputs, Level 3 \| Third Party Pricing Firm \| Nonredeemable preferred stocks \| Industrials
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 6.9		$ 6.9
Fair Value Inputs, Guideline Public Company Method	$ 5.4		$ 5.4
Fair Value, Inputs, Level 3 \| Guideline Public Company Method [Member] \| Nonredeemable preferred stocks \| Industrials
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 9.7	$ 8.1	$ 9.8
Fair Value Inputs, Guideline Public Company Method	$ 4.8	$ 4.0	$ 4.8
Fair Value, Inputs, Level 3 \| Total Quantitative Level Three Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 51.6	$ 28.3	$ 51.7
Fair Value, Inputs, Level 3 \| Third Party Pricing Exemption Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 3.2	$ 0.3	$ 3.1

Weighted average capitalization percentage used to calculate the periodic increase/(decrease) in valuation of Level 3 security based on the Guideline Public Company Method.

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- Definition

Original issue price per share used as market price for current period acquisition. The inputs used to measure this fair value are initially unobservable.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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