Investments - Offsetting of Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Asset derivatives:
Net Amounts of Assets/Liabilities Presented in the Consolidated Balance Sheets	$ 9,376	$ 15,550
Free-standing derivatives
Asset derivatives:
Gross Amounts	9,376	15,550
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts of Assets/Liabilities Presented in the Consolidated Balance Sheets	9,376	15,550
Gross Amounts Not Offset in the Consolidated Balance Sheets, Financial Instruments	0	0
Gross Amounts Not Offset in the Consolidated Balance Sheets, Cash Collateral Received	10,484	15,584
Net Amount	$ (1,108)	$ (34)

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