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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
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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 | $ 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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