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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
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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