Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Net brokered margin gain (losses)					$ (16,300,000)	$ (5,700,000)	$ (2,800,000)
Allowance for doubtful accounts on accounts receivable	$ 6,118,000				6,118,000	7,111,000
Bad debt (income) expense					(1,000,000)	1,550,000	800,000
Inventory and other	22,964,000				22,964,000	21,346,000
Goodwill from MRD merger							1,700,000,000
Purchase adjustment, decrease in goodwill						13,100,000
Goodwill impairment charge	1,641,197,000	$ 0	$ 0	$ 0	1,641,197,000	0	$ 0
Materials and supplies inventory
Summary Of Significant Accounting Policies [Line Items]
Inventory and other	8,000,000				8,000,000	12,100,000
Commodity inventory
Summary Of Significant Accounting Policies [Line Items]
Inventory and other	$ 965,000				$ 965,000	$ 508,000