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Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Details) - Fixed income maturities - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 5,339.9	$ 5,372.6
% of Total Fair Value by Security Type	84.00%	84.00%
U.S. government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 1,577.9	$ 1,551.9
% of Total Fair Value by Security Type	100.00%	100.00%
U.S. agency
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 38.3	$ 45.7
% of Total Fair Value by Security Type	97.00%	96.00%
Municipal
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 15.5	$ 14.7
% of Total Fair Value by Security Type	30.00%	30.00%
Corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 2,755.9	$ 2,775.7
% of Total Fair Value by Security Type	94.00%	95.00%
Non-U.S. government-backed corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 44.1	$ 43.3
% of Total Fair Value by Security Type	45.00%	47.00%
Non-U.S. government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 368.2	$ 366.1
% of Total Fair Value by Security Type	60.00%	56.00%
Asset-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 7.8	$ 7.7
% of Total Fair Value by Security Type	42.00%	39.00%
Agency mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 532.2	$ 567.5
% of Total Fair Value by Security Type	53.00%	53.00%

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