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Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 5,887.6	$ 6,254.6
% of Total Fair Value by Security Type	84.00%	85.00%
Fixed Income Maturities
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 5,887.6	$ 5,763.6
% of Total Fair Value by Security Type	84.00%	84.00%
Fixed Income Maturities | U.S. government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 1,433.9	$ 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	$ 44.4	$ 43.4
% of Total Fair Value by Security Type	84.00%	83.00%
Fixed Income Maturities | Municipal
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 74.7	$ 37.4
% of Total Fair Value by Security Type	61.00%	43.00%
Fixed Income Maturities | Corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 3,275.2	$ 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	$ 42.9	$ 44.0
% of Total Fair Value by Security Type	43.00%	48.00%
Fixed Income Maturities | Non-U.S. government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 373.6	$ 399.4
% of Total Fair Value by Security Type	53.00%	58.00%
Fixed Income Maturities | Asset-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 12.7	$ 13.5
% of Total Fair Value by Security Type	47.00%	37.00%
Fixed Income Maturities | Agency mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 630.2	$ 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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