Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Offsetting Assets [Line Items]
Gross Amounts of Recognized Financial Instruments	$ 191,853	$ 136,775
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Financial Instruments	52,818	25,277
Counterparty A
Offsetting Assets [Line Items]
Gross Amounts of Recognized Financial Instruments	191,853	135,210
Gross Amounts Offset in the Statements of Financial Position	(38,525)	(21,508)
Net Amounts Presented in the Statements of Financial Position	153,328	113,702
Financial Instruments/Collateral	(141,420)	(101,288)
Net Amount	11,908	12,414
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Financial Instruments	52,818	25,277
Gross Amounts Offset in the Statements of Financial Position	(52,818)	(25,277)
Net Amounts Presented in the Statements of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	$ 0	$ 0