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Investments - Troubled Debt Restructuring (Details)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
loan
Dec. 31, 2016
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 1 0
Pre-modification carrying value | $ $ 3.3  
Post-modification carrying value | $ $ 3.3  
Commercial mortgage    
Financing Receivable, Modifications [Line Items]    
Troubled debt restructured loans, number 0 0