Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Net brokered margin gain (losses)	$ (5,700,000)	$ (2,800,000)	$ (2,700,000)
Allowance for doubtful accounts on accounts receivable	7,111,000	5,559,000
Bad debt expense	1,550,000	800,000	$ 2,300,000
Inventory and other	21,346,000	26,573,000
Goodwill	1,641,197,000	1,654,292,000
Materials and supplies inventory
Summary Of Significant Accounting Policies [Line Items]
Inventory and other	12,100,000	9,400,000
Commodity inventory
Summary Of Significant Accounting Policies [Line Items]
Inventory and other	$ 508,000	$ 8,300,000