Offsetting Arrangement (Liabilities Offsetting) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Offsetting Liabilities [Line Items]
Gross amount of Recognized Liabilities	$ 193,781	$ 654,666
Gross amount Offset in the Statement of Financial Condition	0	0
Net Amount of Liabilities Presented in the Statement of Financial Condition	193,781	654,666
Financial Instruments	205,483	700,498
Cash Collateral Provided	1,980	1,980
Net Amount	(13,682)	(47,812)
Derivative Financial Instruments, Liabilities [Member]
Offsetting Liabilities [Line Items]
Gross amount of Recognized Liabilities	1,281	2,437
Gross amount Offset in the Statement of Financial Condition	0	0
Net Amount of Liabilities Presented in the Statement of Financial Condition	1,281	2,437
Financial Instruments	0	0
Cash Collateral Provided	1,980	1,980
Net Amount	(699)	457
Securities Loaned or Sold under Agreements to Repurchase
Offsetting Liabilities [Line Items]
Gross amount of Recognized Liabilities	192,500	652,229
Gross amount Offset in the Statement of Financial Condition	0	0
Net Amount of Liabilities Presented in the Statement of Financial Condition	192,500	652,229
Financial Instruments	205,483	700,498
Cash Collateral Provided	0	0
Net Amount	$ (12,983)	$ (48,269)