Derivative Instruments - Gains (Losses) on Derivative Instruments (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Gain on derivatives, net	$ 12,307,000	$ 18,117,000	$ (42,690,000)	$ (1,371,000)
Total current period settlements on derivatives	12,721,000	922,000	33,515,000	(5,823,000)
Non-trading
Derivative Instruments, Gain (Loss) [Line Items]
Gain on derivatives, net	12,528,000	17,888,000	(42,741,000)	(2,223,000)
Total current period settlements on derivatives	12,764,000	1,035,000	33,677,000	(5,054,000)
Non-trading | Various Acquisitions
Derivative Instruments, Gain (Loss) [Line Items]
Total current period settlements on derivatives	(100,000)	100,000	100,000	(400,000)
Trading
Derivative Instruments, Gain (Loss) [Line Items]
Gain on derivatives, net	(221,000)	229,000	51,000	852,000
Total current period settlements on derivatives	$ (43,000)	$ (113,000)	(162,000)	(769,000)
Trading | Various Acquisitions
Derivative Instruments, Gain (Loss) [Line Items]
Total current period settlements on derivatives			$ (900,000)	$ 0