Fair Value Measurements - Fair Value Amounts by Hierarchy Level (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Liabilities
Gross Fair Value and Nat Carrying Value on Balance Sheet	$ (6,137)	$ (6,808)
Fair Value, Measurements, Recurring
Assets
Gross Fair Value	13,153	26,274
Effect of Counter-Party Netting	(9,936)	(19,650)
Net Carrying Value on Balance Sheet	3,217	6,624
Liabilities
Gross Fair Value	(22,004)	(46,061)
Effect of Counter-Party Netting	9,936	19,650
Net Carrying Value on Balance Sheet	(12,068)	(26,411)
Cash collateral	9,600	7,200
Interest rate derivatives \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	923	3,810
Effect of Counter-Party Netting	0	0
Net Carrying Value on Balance Sheet	923	3,810
Level 1 \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	1,120	557
Liabilities
Gross Fair Value	(2,629)	(596)
Level 1 \| Interest rate derivatives \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	0	0
Level 2 \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	12,033	25,717
Liabilities
Gross Fair Value	(8,816)	(19,093)
Level 2 \| Interest rate derivatives \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	923	3,810
Level 3 \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	0	0
Liabilities
Gross Fair Value	(10,559)	(26,372)
Level 3 \| Interest rate derivatives \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	0	0
Over the Counter \| Fair Value, Measurements, Recurring
Liabilities
Gross Fair Value and Nat Carrying Value on Balance Sheet	(6,137)	(6,808)
Effect of Counter-Party Netting	0	0
Over the Counter \| Level 1 \| Fair Value, Measurements, Recurring
Liabilities
Gross Fair Value and Nat Carrying Value on Balance Sheet	0	0
Over the Counter \| Level 2 \| Fair Value, Measurements, Recurring
Liabilities
Gross Fair Value and Nat Carrying Value on Balance Sheet	0	0
Over the Counter \| Level 3 \| Fair Value, Measurements, Recurring
Liabilities
Gross Fair Value and Nat Carrying Value on Balance Sheet	(6,137)	(6,808)
Exchange Traded \| Future \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	12,230	22,464
Effect of Counter-Party Netting	(9,936)	(19,650)
Net Carrying Value on Balance Sheet	2,294	2,814
Liabilities
Gross Fair Value	(11,445)	(19,689)
Effect of Counter-Party Netting	9,936	19,650
Net Carrying Value on Balance Sheet	(1,509)	(39)
Exchange Traded \| J. Aron repurchase obligation derivative \| Fair Value, Measurements, Recurring
Liabilities
Gross Fair Value	(4,422)	(19,564)
Effect of Counter-Party Netting	0	0
Net Carrying Value on Balance Sheet	(4,422)	(19,564)
Exchange Traded \| Level 1 \| Future \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	1,120	557
Liabilities
Gross Fair Value	(2,629)	(596)
Exchange Traded \| Level 1 \| J. Aron repurchase obligation derivative \| Fair Value, Measurements, Recurring
Liabilities
Gross Fair Value	0	0
Exchange Traded \| Level 2 \| Future \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	11,110	21,907
Liabilities
Gross Fair Value	(8,816)	(19,093)
Exchange Traded \| Level 2 \| J. Aron repurchase obligation derivative \| Fair Value, Measurements, Recurring
Liabilities
Gross Fair Value	0	0
Exchange Traded \| Level 3 \| Future \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	0	0
Liabilities
Gross Fair Value	0	0
Exchange Traded \| Level 3 \| J. Aron repurchase obligation derivative \| Fair Value, Measurements, Recurring
Liabilities
Gross Fair Value	$ (4,422)	$ (19,564)

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Amount of right to receive cash collateral under master netting arrangements offset against derivative liabilities.

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Value of outstanding derivative securities that permit the holder the right to purchase securities (usually equity) from the issuer at a specified price.

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