Postretirement Medical Plan (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Postretirement Medical Plan
Net periodic benefit cost				$ (4,103)	$ (423)
Post retirement medical plan
Postretirement Medical Plan
Service cost	$ 223	$ 269		670	1,956
Interest cost	230	185		689	1,002
Amortization of prior service cost	(1,821)	(1,872)		(5,462)	(3,381)
Net periodic benefit cost	$ (1,368)	$ (1,418)		$ (4,103)	$ (423)
Negative prior service cost base to be amortized			$ (41,100)
Reduction in accumulated postretirement benefit obligation			$ (47,700)
Postretirement Credit Amortization Period			4 years 2 months 12 days