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Fair Value Disclosure - Reconciliation of Level 3 Investments (Details) - Commercial Mortgage Backed Securities [Member]
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]  
Fair value at December 31, 2016 $ 0
Transfers into Level III 254,484
Net realized loss on investments (41,651)
Net increase in unrealized gain on investments 38,830
Sales and repayments of investments (250,464)
Amortization of purchase discount, net (1,199)
Fair value at December 31, 2017 $ 0