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Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 5,372.6	$ 6,254.6
% of Total Fair Value from Index Providers	84.00%	85.00%
U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 1,551.9	$ 1,321.3
% of Total Fair Value from Index Providers	100.00%	100.00%
U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 45.7	$ 43.4
% of Total Fair Value from Index Providers	96.00%	83.00%
Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 14.7	$ 37.4
% of Total Fair Value from Index Providers	30.00%	43.00%
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 2,775.7	$ 3,299.6
% of Total Fair Value from Index Providers	95.00%	83.00%
Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 43.3	$ 44.0
% of Total Fair Value from Index Providers	47.00%	48.00%
Non-U.S government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 366.1	$ 399.4
% of Total Fair Value from Index Providers	56.00%	58.00%
Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 7.7	$ 13.5
% of Total Fair Value from Index Providers	39.00%	37.00%
Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 567.5	$ 605.0
% of Total Fair Value from Index Providers	53.00%	53.00%
Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 5,372.6	$ 5,763.6
% of Total Fair Value from Index Providers	84.00%	84.00%
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 0.0	$ 491.0
% of Total Fair Value from Index Providers	0.00%	100.00%

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