Fair Value Measurements - Fair Value Amounts by Hierarchy Level (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Liabilities
Gross Fair Value and Nat Carrying Value on Balance Sheet	$ (7,345)	$ (5,134)
Fair Value, Measurements, Recurring
Assets
Gross Fair Value	31,934	29,887
Effect of Counter-Party Netting	(26,836)	(23,601)
Net Carrying Value on Balance Sheet	5,098	6,286
Liabilities
Gross Fair Value	(59,234)	(49,424)
Effect of Counter-Party Netting	26,836	23,601
Net Carrying Value on Balance Sheet	(32,398)	(25,823)
Cash collateral	7,700	9,700
Interest rate derivatives \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	2,966	3,602
Effect of Counter-Party Netting	0	(64)
Net Carrying Value on Balance Sheet	2,966	3,538
Liabilities
Gross Fair Value		(158)
Effect of Counter-Party Netting		64
Net Carrying Value on Balance Sheet		(94)
Level 1 \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	235	190
Liabilities
Gross Fair Value	(150)	(54)
Level 1 \| Interest rate derivatives \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	0	0
Liabilities
Gross Fair Value		0
Level 2 \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	31,699	29,697
Liabilities
Gross Fair Value	(26,744)	(24,236)
Level 2 \| Interest rate derivatives \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	2,966	3,602
Liabilities
Gross Fair Value		(158)
Level 3 \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	0	0
Liabilities
Gross Fair Value	(32,340)	(25,134)
Level 3 \| Interest rate derivatives \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	0	0
Liabilities
Gross Fair Value		0
Over the Counter \| Fair Value, Measurements, Recurring
Liabilities
Gross Fair Value and Nat Carrying Value on Balance Sheet	(7,345)	(5,134)
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	(7,345)	(5,134)
Exchange Traded \| Future \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	28,968	26,285
Effect of Counter-Party Netting	(26,836)	(23,537)
Net Carrying Value on Balance Sheet	2,132	2,748
Liabilities
Gross Fair Value	(26,894)	(24,132)
Effect of Counter-Party Netting	26,836	23,537
Net Carrying Value on Balance Sheet	(58)	(595)
Exchange Traded \| J. Aron repurchase obligation derivative \| Fair Value, Measurements, Recurring
Liabilities
Gross Fair Value	(24,995)	(20,000)
Effect of Counter-Party Netting	0	0
Net Carrying Value on Balance Sheet	(24,995)	(20,000)
Exchange Traded \| Level 1 \| Future \| Fair Value, Measurements, Recurring
Assets
Gross Fair Value	235	190
Liabilities
Gross Fair Value	(150)	(54)
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	28,733	26,095
Liabilities
Gross Fair Value	(26,744)	(24,078)
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	$ (24,995)	$ (20,000)

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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