Postretirement Medical Plan (Details) - Post retirement medical plan - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Defined Benefit Plan Net Periodic Benefit Cost [Abstract]
Service cost	$ 1,229	$ 1,038	$ 2,458	$ 2,075
Interest cost	482	464	964	929
Amortization of prior service cost	313	247	626	494
Net periodic benefit cost	$ 2,024	$ 1,749	$ 4,048	$ 3,498