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Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 6,254.6	$ 6,524.1
% of Total Fair Value from Index Providers	85.00%	88.00%
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	$ 1,321.3	$ 1,288.2
% 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	$ 43.4	$ 110.2
% of Total Fair Value from Index Providers	83.00%	92.00%
Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 37.4	$ 28.8
% of Total Fair Value from Index Providers	43.00%	72.00%
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 3,299.6	$ 3,275.3
% of Total Fair Value from Index Providers	83.00%	96.00%
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	$ 44.0	$ 44.8
% of Total Fair Value from Index Providers	48.00%	50.00%
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	$ 399.4	$ 455.6
% of Total Fair Value from Index Providers	58.00%	72.00%
Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 13.5	$ 32.1
% of Total Fair Value from Index Providers	37.00%	41.00%
Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 0.0	$ 12.5
% of Total Fair Value from Index Providers	0.00%	98.00%
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	$ 605.0	$ 691.9
% of Total Fair Value from Index Providers	53.00%	58.00%
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	$ 5,763.6	$ 5,939.4
% of Total Fair Value from Index Providers	84.00%	87.00%
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 491.0	$ 584.7
% of Total Fair Value from Index Providers	100.00%	100.00%

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