Amount of Commitment Expiration Per Period (Details) $ in Thousands	Sep. 30, 2018 USD ($)
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	$ 276,724
Less Than 1 Year [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	93,847
1-3 Years [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	182,877
3-5 Years [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Beyond 5 Years [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Standby Letters of Credit [Member] | Firm Transportation [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	251,057
Standby Letters of Credit [Member] | Other Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	285
Standby Letters of Credit [Member] | Loss Contingencies by Secondary Nature of Contingency [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	251,342
Standby Letters of Credit [Member] | Less Than 1 Year [Member] | Firm Transportation [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	69,538
Standby Letters of Credit [Member] | Less Than 1 Year [Member] | Other Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	265
Standby Letters of Credit [Member] | Less Than 1 Year [Member] | Loss Contingencies by Secondary Nature of Contingency [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	69,803
Standby Letters of Credit [Member] | 1-3 Years [Member] | Firm Transportation [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	181,519
Standby Letters of Credit [Member] | 1-3 Years [Member] | Other Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	20
Standby Letters of Credit [Member] | 1-3 Years [Member] | Loss Contingencies by Secondary Nature of Contingency [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	181,539
Standby Letters of Credit [Member] | 3-5 Years [Member] | Firm Transportation [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Standby Letters of Credit [Member] | 3-5 Years [Member] | Other Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Standby Letters of Credit [Member] | 3-5 Years [Member] | Loss Contingencies by Secondary Nature of Contingency [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Standby Letters of Credit [Member] | Beyond 5 Years [Member] | Firm Transportation [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Standby Letters of Credit [Member] | Beyond 5 Years [Member] | Other Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Standby Letters of Credit [Member] | Beyond 5 Years [Member] | Loss Contingencies by Secondary Nature of Contingency [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Surety Bond [Member] | Other Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	12,396
Surety Bond [Member] | Employee Related Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	1,850
Surety Bond [Member] | Environment Related Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	11,136
Surety Bond [Member] | Loss Contingencies by Secondary Nature of Contingency [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	25,382
Surety Bond [Member] | Less Than 1 Year [Member] | Other Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	11,523
Surety Bond [Member] | Less Than 1 Year [Member] | Employee Related Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	1,500
Surety Bond [Member] | Less Than 1 Year [Member] | Environment Related Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	11,021
Surety Bond [Member] | Less Than 1 Year [Member] | Loss Contingencies by Secondary Nature of Contingency [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	24,044
Surety Bond [Member] | 1-3 Years [Member] | Other Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	873
Surety Bond [Member] | 1-3 Years [Member] | Employee Related Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	350
Surety Bond [Member] | 1-3 Years [Member] | Environment Related Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	115
Surety Bond [Member] | 1-3 Years [Member] | Loss Contingencies by Secondary Nature of Contingency [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	1,338
Surety Bond [Member] | 3-5 Years [Member] | Other Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Surety Bond [Member] | 3-5 Years [Member] | Employee Related Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Surety Bond [Member] | 3-5 Years [Member] | Environment Related Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Surety Bond [Member] | 3-5 Years [Member] | Loss Contingencies by Secondary Nature of Contingency [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Surety Bond [Member] | Beyond 5 Years [Member] | Other Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Surety Bond [Member] | Beyond 5 Years [Member] | Employee Related Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Surety Bond [Member] | Beyond 5 Years [Member] | Environment Related Commitments [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	0
Surety Bond [Member] | Beyond 5 Years [Member] | Loss Contingencies by Secondary Nature of Contingency [Member]
Schedule of Costs Related to Purchase Obligations [Line Items]
Guarantor obligations, maximum exposure	$ 0