Fair Value of Financial Instruments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfer to Level 3	$ 62.5	$ 110.6
Transfer out of level 3	$ 182.0	$ 5.7
Level 3 | Assets, Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of total assets (less than)	1.00%
Structured securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfer to Level 3	$ 119.5
Transfer out of level 3	$ 104.9