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Investments - Narrative (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]      
Equity Securities, FV-NI, Unrealized Gain (Loss) $ 63,400,000    
Other Receivables 254,600,000 $ 245,400,000  
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date 1,178,100,000    
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date 1,179,600,000    
Other Liabilities 742,700,000 687,300,000  
Available-for-sale-Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Securities that have had Recognized Credit Losses in Earnings, Aggregate Losses   0 $ 0
Available For Sale Securities, Continuous Unrealized Loss Position, Credit Losses Recognized Related to Securities in An Unrealized Loss Position, In a Position of 12 Months or Longer 0   $ 0
Equity Securities without Readily Determinable Fair Value, Impairment Loss, Annual Amount 200,000    
Equity Securities without Readily Determinable Fair Value, Impairment Loss, Cumulative Amount   5.1  
Investments in Fixed Maturities      
Debt Securities, Available-for-sale [Line Items]      
Less Than 12 Months 648,400,000 2,470,800,000  
Total 34,600,000 86,600,000  
12 Months or Longer 22,400,000 22,000,000.0  
Other Liabilities 15,500,000 10,500,000  
Less Than 12 Months 12,200,000 64,600,000  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 695,200,000 302,900,000  
12 Months or Longer 22,400,000 22,000,000.0  
Debt Securities, Available-for-sale, Unrealized Loss Position 1,343,600,000 2,773,700,000  
Total 34,600,000 86,600,000  
U.S. Government and Government Agencies and Authorities      
Debt Securities, Available-for-sale [Line Items]      
Less Than 12 Months 13,200,000 401,100,000  
Less Than 12 Months 200,000 7,600,000  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 262,200,000 79,000,000.0  
12 Months or Longer 7,500,000 7,400,000  
Debt Securities, Available-for-sale, Unrealized Loss Position 275,400,000 480,100,000  
Total 7,700,000 15,000,000.0  
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]      
Debt Securities, Available-for-sale [Line Items]      
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date 616,400,000    
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]      
Debt Securities, Available-for-sale [Line Items]      
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date 8,100,000    
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]      
Debt Securities, Available-for-sale [Line Items]      
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date 7,600,000    
Other Non-Governmental Issuers [Member]      
Debt Securities, Available-for-sale [Line Items]      
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date 547,500,000    
Equity Securities [Member]      
Debt Securities, Available-for-sale [Line Items]      
Other Receivables 0 0  
Other Liabilities   0  
External Credit Rating, Investment Grade [Member] | Investments in Fixed Maturities      
Debt Securities, Available-for-sale [Line Items]      
Total 23,100,000 69,500,000  
External Credit Rating, Non Investment Grade [Member] | Investments in Fixed Maturities      
Debt Securities, Available-for-sale [Line Items]      
Total $ 11,500,000 $ 17,100,000  
Percentage of Unrealized Loss Position to Amortized Cost Basis of Available for Sale Security Average 4.00%   5.00%