Derivatives - Offsetting Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Offsetting Assets [Line Items]
Gross Amounts of Assets Presented in the Consolidated Balance Sheet	$ 1,665	$ 999
Derivatives, at fair value	1,665	999
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheet, Financial Instruments	(298,685)	(233,301)
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheet, Net Cash Collateral Held	304,396	241,604
Net Amount	7,376	9,302
Interest rate swap contracts
Offsetting Assets [Line Items]
Gross Amounts of Assets Presented in the Consolidated Balance Sheet	0	457
Derivatives, at fair value	0	457
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheet, Financial Instruments	(298,685)	(233,301)
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheet, Net Cash Collateral Held	304,396	241,604
Net Amount	5,711	8,760
Basis swap contracts
Offsetting Assets [Line Items]
Gross Amounts of Assets Presented in the Consolidated Balance Sheet	1,665	542
Derivatives, at fair value	1,665	542
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheet, Financial Instruments	0	0
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheet, Net Cash Collateral Held	0	0
Net Amount	$ 1,665	$ 542