Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) Segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Net brokered margin gain (loss)	$ (2,800)	$ (2,700)	$ 9,400
Allowance for doubtful accounts on accounts receivable	5,559	4,994
Bad debt expense	800	2,300	$ 250
Inventory, other	$ 26,573	33,217
Number of reporting unit | Segment	1
Materials and supplies inventory
Summary Of Significant Accounting Policies [Line Items]
Inventory, other	$ 9,400	20,800
Commodity inventory
Summary Of Significant Accounting Policies [Line Items]
Inventory, other	$ 8,300	$ 4,800