Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Offsetting Assets [Line Items]
Gross Amounts of Recognized Financial Instruments	$ 9,039,223	$ 136,775
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Financial Instruments	1,385,288	25,277
Counterparty A
Offsetting Assets [Line Items]
Gross Amounts of Recognized Financial Instruments	9,037,140	135,210
Gross Amounts Offset in the Statement of Financial Position	(9,037,140)	(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	1,325,365	25,277
Gross Amounts Offset in the Statement of Financial Position	(1,108,829)	(25,277)
Net Amounts Presented in the Statement of Financial Position	216,536	0
Financial Instruments/Collateral	(216,536)	0
Net Amount	$ 0	$ 0