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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
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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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