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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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