Derivative Instruments (Offsetting Balance Sheet) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Offsetting of Financial Assets, Derivatives
Gross Amounts of Recognized Financial Instruments	$ 90,884	$ 300,655
Securities purchased under agreements to resell
Gross Amounts of Recognized Financial Instruments	58,366	156,064
Gross Amounts Offset in the Consolidated Statement of Financial Position	0	0
Net Amounts Presented in the Consolidated Statement of Financial Position	58,366	156,064
Financial Instruments/Collateral	(58,366)	(156,064)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	149,243	453,435
Gross Amounts Offset in the Consolidated Statement of Financial Position	(13,019)	(230,554)
Net Amounts Presented in the Consolidated Statement of Financial Position	136,224	222,881
Financial Instruments/Collateral	(136,224)	(171,221)
Net Amount	0	51,660
Offsetting of Financial Liabilities, Derivatives
Gross Amounts of Recognized Financial Instruments	12,854	32,849
Securities sold under agreements to repurchase
Gross Amounts of Recognized Financial Instruments	68,904	0
Gross Amounts Offset in the Consolidated Statement of Financial Position	0	0
Net Amounts Presented in the Consolidated Statement of Financial Position	68,904	0
Financial Instruments/Collateral	(68,904)	0
Net Amount	0	0
Total Liabilities
Gross Amounts of Recognized Financial Instruments	81,758	32,849
Gross Amounts Offset in the Consolidated Statement of Financial Position	(12,854)	(32,849)
Net Amounts Presented in the Consolidated Statement of Financial Position	68,904	0
Financial Instruments/Collateral	(68,904)	0
Net Amount	0	0
Counterparty
Offsetting of Financial Assets, Derivatives
Gross Amounts of Recognized Financial Instruments	90,877	297,371
Gross Amounts Offset in the Consolidated Statement of Financial Position	(13,019)	(230,554)
Net Amounts Presented in the Consolidated Statement of Financial Position	77,858	66,817
Financial Instruments/Collateral	(77,858)	(15,157)
Net Amount	0	51,660
Offsetting of Financial Liabilities, Derivatives
Gross Amounts of Recognized Financial Instruments	12,854	32,849
Gross Amounts Offset in the Consolidated Statement of Financial Position	(12,854)	(32,849)
Net Amounts Presented in the Consolidated Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	$ 0	$ 0