Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Offsetting Assets [Line Items]
Gross Amounts of Recognized Financial Instruments	$ 12,678,048	$ 136,775
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Financial Instruments	3,625,754	25,277
Counterparty A
Offsetting Assets [Line Items]
Gross Amounts of Recognized Financial Instruments	12,678,048	135,210
Gross Amounts Offset in the Statement of Financial Position	(12,678,048)	(21,508)
Net Amounts Presented in the Statement of Financial Position	0	113,702
Financial Instruments/Collateral	0	(101,288)
Net Amount	0	12,414
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Financial Instruments	3,557,366	25,277
Gross Amounts Offset in the Statement of Financial Position	(2,695,145)	(25,277)
Net Amounts Presented in the Statement of Financial Position	862,221	0
Financial Instruments/Collateral	(700,351)	0
Net Amount	$ 161,870	$ 0