Derivative Instruments (Offsetting Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Offsetting of Financial Assets:
Gross Amounts of Recognized Financial Instruments	$ 136,775	$ 118,846
Offsetting of Financial Liabilities:
Gross Amounts of Recognized Financial Instruments	25,277	21,249
Counterparty
Offsetting of Financial Assets:
Gross Amounts of Recognized Financial Instruments	135,210	118,846
Gross Amounts Offset in the Statement of Financial Position	(21,508)	(24,288)
Net Amounts Presented in the Statement of Financial Position	113,702	94,558
Financial Instruments / Collateral	(101,288)	(82,602)
Net Amount	12,414	11,956
Offsetting of Financial Liabilities:
Gross Amounts of Recognized Financial Instruments	25,277	21,249
Gross Amounts Offset in the Statement of Financial Position	(25,277)	(21,249)
Net Amounts Presented in the Statement of Financial Position	0	0
Financial Instruments / Collateral	0	0
Net Amount	$ 0	$ 0