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Investments - Troubled Debt Restructuring (Details)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
loan
Mar. 31, 2017
loan
Dec. 31, 2017
USD ($)
loan
Financing Receivable, Modifications [Line Items]      
Troubled debt restructured loans, subsequent payment default, number 0 0  
Private placement debt      
Financing Receivable, Modifications [Line Items]      
Troubled debt restructured loans, number   0 0
Pre-modification carrying value | $     $ 3.0
Post-modification carrying value | $ $ 3.3    
Commercial mortgage      
Financing Receivable, Modifications [Line Items]      
Troubled debt restructured loans, number 0 1