Derivative Instruments (Tables)

12 Months Ended

Dec. 31, 2017

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Schedule of impact of netting agreements and margin deposits on gross derivative assets and liabilities

The table below reflects the impact of netting agreements and margin deposits on gross derivative assets and liabilities as of December 31, 2017 and 2016.

As of December 31, 2017	Derivative instruments, recorded in the Consolidated Balance Sheet, gross		Derivative instruments subject to master netting agreements			Margin deposits remitted to counterparties		Derivative instruments, net
	(Thousands)
Asset derivatives:
Derivative instruments, at fair value	$	241,952	$	(86,856	)	$	—	$	155,096
Liability derivatives:
Derivative instruments, at fair value	$	139,089	$	(86,856	)	$	—	$	52,233

As of December 31, 2016	Derivative instruments, recorded in the Consolidated Balance Sheet, gross		Derivative instruments subject to master netting agreements			Margin deposits remitted to counterparties		Derivative instruments, net
	(Thousands)
Asset derivatives:
Derivative instruments, at fair value	$	33,053	$	(23,373	)	$	—	$	9,680
Liability derivatives:
Derivative instruments, at fair value	$	257,943	$	(23,373	)	$	—	$	234,570