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Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 6,338.0 | $ 6,524.1
% of Total Fair Value by Security Type | 85.00% | 88.00%
Fixed Income Maturities |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 5,869.4 | $ 5,939.4
% of Total Fair Value by Security Type | 84.00% | 87.00%
Fixed Income Maturities | U.S. government |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 1,333.2 | $ 1,288.2
% 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 | $ 53.2 | $ 110.2
% of Total Fair Value by Security Type | 86.00% | 92.00%
Fixed Income Maturities | Municipal |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 31.2 | $ 28.8
% of Total Fair Value by Security Type | 38.00% | 72.00%
Fixed Income Maturities | Corporate |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 3,346.0 | $ 3,275.3
% of Total Fair Value by Security Type | 95.00% | 96.00%
Fixed Income Maturities | Non-U.S. government-backed corporate |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 47.8 | $ 44.8
% of Total Fair Value by Security Type | 50.00% | 50.00%
Fixed Income Maturities | Non-U.S. government |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 425.6 | $ 455.6
% of Total Fair Value by Security Type | 61.00% | 72.00%
Fixed Income Maturities | Asset-backed |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 14.2 | $ 32.1
% of Total Fair Value by Security Type | 32.00% | 41.00%
Fixed Income Maturities | Non-agency commercial mortgage-backed |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 0.0 | $ 12.5
% of Total Fair Value by Security Type | 0.00% | 98.00%
Fixed Income Maturities | Agency mortgage-backed |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 618.2 | $ 691.9
% of Total Fair Value by Security Type | 54.00% | 58.00%
Equity securities — Trading |  |
Fair Value [Line Items] |  |
Fair Market Value Determined using Prices from Index Providers | $ 468.6 | $ 584.7
% of Total Fair Value by Security Type | 100.00% | 100.00%

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