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Commitments and Contingencies - Schedule of Company's Restricted Assets (Details)
£ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2017
GBP (£)
Dec. 31, 2016
GBP (£)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]        
Total restricted assets $ 4,638.8 $ 4,535.7    
Total as percent of investable assets 53.40% 49.30%    
Securities and cash as collateral secured letters of credit $ 658.5 $ 672.0 £ 0.0 £ 7.2
Affiliated transactions        
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]        
Total restricted assets 1,455.0 1,482.8    
Third party        
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]        
Total restricted assets 2,425.3 2,380.8    
Letters of credit / guarantees        
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]        
Total restricted assets 658.5 672.1    
Real estate fund        
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]        
Total restricted assets $ 100.0 $ 0.0