Investments - Offsetting of Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Asset derivatives:
Net Amounts of Assets/Liabilities Presented in the Consolidated Balance Sheets	$ 10,198	$ 15,550
Free-standing derivatives
Asset derivatives:
Gross Amounts	10,198	15,550
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts of Assets/Liabilities Presented in the Consolidated Balance Sheets	10,198	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	11,638	15,584
Net Amount	$ (1,440)	$ (34)