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Fair Value Summary of Quantitative Information about Level 3 Fair Value Measurements (Details)
$ / shares in Units, $ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2020
USD ($)
$ / shares
Sep. 30, 2019
USD ($)
$ / shares
Dec. 31, 2019
USD ($)
$ / shares
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Investments, Fair Value $ 45,764.3 $ 38,573.7 $ 39,254.3
Fair Value, Inputs, Level 3      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Investments, Fair Value 56.0 32.4 43.4
Fair Value, Inputs, Level 3 | Guideline Public Company Method [Member] | Nonredeemable preferred stocks | Financials      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Investments, Fair Value $ 13.1    
Fair Value Inputs, Guideline Public Company Method | $ / shares $ (51.5)    
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 $ 15.2    
Fair Value Inputs, Guideline Public Company Method | $ / shares $ (19.2)    
Fair Value, Inputs, Level 3 | Third Party Pricing Firm | Nonredeemable preferred stocks | Financials      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Investments, Fair Value     $ 27.1
Fair Value Inputs, Recent Transaction Price | $ / shares     $ 9.0
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     $ 6.0
Fair Value Inputs, Performance Based Price Adjustment | $ / shares     $ 4.8
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 $ 27.1  
Fair Value Inputs, Original Issue Price | $ / shares $ 3.7 $ 9.0  
Fair Value, Inputs, Level 3 | Internal Pricing | Nonredeemable preferred stocks | Industrials      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Investments, Fair Value   $ 5.0 $ 10.0
Fair Value Inputs, Price-to-Sales Ratio   5.5  
Fair Value Inputs, Original Issue Price | $ / shares     $ 4.9
Fair Value, Inputs, Level 3 | Total Quantitative Level Three Securities      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Investments, Fair Value $ 53.3 $ 32.1 $ 43.1
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Investments, Fair Value $ 2.7 $ 0.3 $ 0.3