Commitments and Contingencies - Other Commitments (Details) $ in Thousands	1 Months Ended			3 Months Ended	12 Months Ended
	Jun. 30, 2014 USD ($)	May 31, 2014 USD ($)	Apr. 30, 2014 USD ($) mi	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Pipeline Connection Agreement [Member] | Divide County, North Dakota to Columbus, North Dakota [Member]
Other Commitment
Commitment amount	$ 55,000				$ 32,500
Period under the agreement	7 years
Pipeline Connection Agreement [Member] | Dunn and McKenzie Counties, North Dakota to Williams County, North Dakota [Member]
Other Commitment
Commitment amount		$ 38,200			15,200
Minimum quarterly commitment amount					1,400
Period under the agreement		7 years
Pipeline Connection Agreement [Member] | Mercer Country, North Dakota to Beulah, North Dakota [Member]
Other Commitment
Commitment amount			$ 14,600		0
Period under the agreement			5 years
Length of pipeline lateral which would be built, owned and operated | mi			4
Natural Gas Transportation and Reservation Agreement [Member]
Other Commitment
Commitment amount					$ 5,800
Period under the agreement					5 years
Pipeline connection agreement with Tesoro Logistics
Other Commitment
Ownership interest					60.00%
Payment to accelerate and terminate agreement				$ 13,100
Rail Spur And Dock Access Right Agreements [Member]
Other Commitment
Commitment amount					$ 31,500
Expense under agreements					$ 900	$ 1,000	$ 800