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, 2019 USD ($) $ / shares	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 38,573.7	$ 32,350.6	$ 33,567.4
Fair Value, Inputs, Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	32.4	5.3	30.4
Fair Value, Inputs, Level 3 | Internal Pricing | Nonredeemable preferred stocks
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	32.1	5.0	30.1
Fair Value, Inputs, Level 3 | Internal Pricing | Nonredeemable preferred stocks | Financials
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 27.1	0.0	$ 25.1
Fair Value Inputs, Original Issue Price | $ / shares	$ 9.0		$ 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	$ 5.0	$ 5.0
Fair Value Inputs, Price-to-Sales Ratio	5.5	3.9	5.5
Fair Value, Inputs, Level 3 | Total Quantitative Level Three Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 32.1	$ 5.0	$ 30.1
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 0.3	$ 0.3	$ 0.3