Employee Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Mar. 31, 2015
Other Postretirement Benefits
Settlement loss	$ 1,400		$ 2,200
Other comprehensive income due to the results of fair value of plan assets	3,900		$ 1,700
Actuarial assumptions:
Discount rate (as a percent)			4.09%
Union employees
Other Postretirement Benefits
Lump-sum benefits to plan participants in full settlement of obligations	13,700		$ 19,700
Non-union employees
Other Postretirement Benefits
Lump-sum benefits to plan participants in full settlement of obligations			600
Minimum
Other Postretirement Benefits
Discount rate used to measure the plan obligations (as percent)					3.54%
Maximum
Other Postretirement Benefits
Discount rate used to measure the plan obligations (as percent)					3.57%
Pension
Other Postretirement Benefits
Interest cost	1,984	$ 2,208	4,057	$ 4,416
Expected asset return	(3,366)	(3,178)	(6,760)	(6,356)
Amortization of loss	39	29	(18)	58
Net periodic benefit income	(1,343)	(941)	(2,721)	(1,882)
Settlement loss	1,397		2,248
Net benefit expense (income)	54	(941)	(473)	(1,882)
Expected company contribution			10,000
Company contribution			4,200
Other Postretirement Benefits
Other Postretirement Benefits
Service cost	259	230	518	460
Interest cost	589	602	1,178	1,204
Amortization of loss	149	15	299	30
Net periodic benefit income	$ 997	$ 847	$ 1,995	$ 1,694