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Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
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Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 5,562.6 | $ 6,254.6
% of Total Fair Value by Security Type | 85.00% | 85.00%
Fixed income maturities |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 5,562.6 | $ 5,763.6
% of Total Fair Value by Security Type | 85.00% | 84.00%
Fixed income maturities | U.S. government |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 1,553.6 | $ 1,321.3
% of Total Fair Value by Security Type | 100.00% | 100.00%
Fixed income maturities | U.S. agency |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 44.6 | $ 43.4
% of Total Fair Value by Security Type | 96.00% | 83.00%
Fixed income maturities | Municipal |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 22.3 | $ 37.4
% of Total Fair Value by Security Type | 31.00% | 43.00%
Fixed income maturities | Corporate |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 2,980.9 | $ 3,299.6
% of Total Fair Value by Security Type | 95.00% | 83.00%
Fixed income maturities | Non-U.S. government-backed corporate |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 40.0 | $ 44.0
% of Total Fair Value by Security Type | 44.00% | 48.00%
Fixed income maturities | Non-U.S. government |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 360.4 | $ 399.4
% of Total Fair Value by Security Type | 57.00% | 58.00%
Fixed income maturities | Asset-backed |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 7.7 | $ 13.5
% of Total Fair Value by Security Type | 33.00% | 37.00%
Fixed income maturities | Agency mortgage-backed |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 553.1 | $ 605.0
% of Total Fair Value by Security Type | 54.00% | 53.00%
Equity securities — Trading |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 0.0 | $ 491.0
% of Total Fair Value by Security Type | 0.00% | 100.00%

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