Employee Benefit Plans - Components of Net Periodic Benefit Cost and Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Mar. 31, 2014
Pension Plan [Member]
Components of net periodic benefit cost
Service cost	$ 97	$ 97
Interest cost	1,546	1,415
Expected return on plan assets	(1,710)	(1,823)
Amortization of net loss(1)	370	303
Net periodic benefit cost (credit)	$ 303	$ (8)
Weighted average assumptions used to determine benefit obligations
Discount rate	4.40%	4.15%
Expected long-term return on plan assets	7.50%	7.50%
Other Postretirement Benefit Plan, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost	$ 347	$ 234
Interest cost	299	202
Expected return on plan assets	0	0
Amortization of net loss(1)	(49)	(220)
Amortization of prior service cost (credit)	(439)	(440)
Net periodic benefit cost (credit)	$ 158	$ (224)
Retiree Medical Plan
Weighted average assumptions used to determine benefit obligations
Discount rate	4.69%		4.29%
Death Benefit Plan
Weighted average assumptions used to determine benefit obligations
Discount rate	4.74%		4.48%