Fair Value Summary of Quantitative Information about Level 3 Fair Value Measurements (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 52,287.4	$ 45,764.3	$ 47,530.3
Fair Value, Inputs, Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	177.9	56.0	54.8
Fair Value, Inputs, Level 3 | Total Quantitative Level Three Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	165.3	53.3	51.7
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	12.6	2.7	3.1
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Financials | Recent Transaction Price, Twenty Seven Point Five Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 16.2
Fair Value Inputs, Recent Transaction Price	$ 27.5
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Financials | Unadjusted Purchase Price, Twenty Point One Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 22.5
Fair Value Inputs, Original Issue Price	$ 20.1
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Financials | Unadjusted Purchase Price, Eleven Point Eight Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 12.5
Fair Value Inputs, Original Issue Price	$ 11.8
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Financials | Unadjusted Purchase Price, Thirty Two Point One Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 10.2
Fair Value Inputs, Original Issue Price	$ 32.1
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Financials | Unadjusted Purchase Price, Six Point Eight Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 15.0
Fair Value Inputs, Original Issue Price	$ 6.8
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Financials | Guideline Public Company Method, Four Point Four Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value		$ 13.1
Fair Value Inputs, Guideline Public Company Method		$ 4.4
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Financials | Unadjusted Purchase Price, Sixteen Point Nine Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value			$ 10.0
Fair Value Inputs, Original Issue Price			$ 16.9
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Industrials | Recent Transaction Price, Five Point Two Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 10.7
Fair Value Inputs, Recent Transaction Price	$ 5.2
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Industrials | Recent Transaction Price, Nine Point Six Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 23.7
Fair Value Inputs, Recent Transaction Price	$ 9.6
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Industrials | Guideline Public Company Method, Five Point Six Dollars per Share
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 | Nonredeemable preferred stocks | Industrials | Guideline Public Company Method, Four Point Zero Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value		$ 8.1
Fair Value Inputs, Guideline Public Company Method		$ 4.0
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Industrials | Guideline Public Company Method, Five Point Four Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value			$ 6.9
Fair Value Inputs, Guideline Public Company Method			$ 5.4
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Industrials | Guideline Public Company Method, Four Point Eight Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value			$ 9.8
Fair Value Inputs, Guideline Public Company Method			$ 4.8
Fair Value, Inputs, Level 3 | Common equities | Guideline Public Company Method, Seven Point Seven Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 54.5
Fair Value Inputs, Recent Transaction Price	$ 7.7
Fair Value, Inputs, Level 3 | Common equities | Unadjusted Purchase Price, Three Point Seven Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value		$ 25.0	$ 25.0
Fair Value Inputs, Original Issue Price		$ 3.7	$ 3.7