Investments - Offsetting of Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Asset derivatives:
Net Amounts of Assets/Liabilities Presented in the Consolidated Balance Sheets	$ 15,550	$ 8,694
Free-standing derivatives
Asset derivatives:
Gross Amounts	15,550	8,694
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts of Assets/Liabilities Presented in the Consolidated Balance Sheets	15,550	8,694
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	15,584	8,824
Net Amount	$ (34)	$ (130)