Employee Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jan. 31, 2017	Dec. 31, 2016
Pension and Other Postretirement Benefits
Total benefit (income) expense	$ (830)	$ (320)	$ (2,318)	$ (1,426)
Settlement loss	1,090	486	3,360	486
Pension
Pension and Other Postretirement Benefits
Interest cost	1,626	1,703	5,126	5,695
Expected asset return	(2,576)	(2,159)	(7,805)	(7,513)
Amortization of loss	120	136	361	392
Total benefit (income) expense	260	$ 166	1,042	$ (940)
Discount rate used to measure the plan obligations (as percent)		3.28%		3.28%
Other comprehensive gain (loss) due to the results of fair value of plan assets	(2,700)	$ (8,900)	(3,500)	$ (8,900)
Expected company contribution						$ 4,800
Company contribution			3,000
Pension | Union employees
Pension and Other Postretirement Benefits
Lump-sum benefits to plan participants in full settlement of obligations	$ 6,800		$ 19,100	2,400
Pension | Minimum
Pension and Other Postretirement Benefits
Discount rate used to measure the plan obligations (as percent)	3.56%		3.56%
Pension | Maximum
Pension and Other Postretirement Benefits
Discount rate used to measure the plan obligations (as percent)	3.57%		3.57%
Other Postretirement Benefits
Pension and Other Postretirement Benefits
Service cost	$ 215	259	$ 683	777
Interest cost	532	655	1,661	1,964
Amortization of loss	(213)	119	(464)	356
Total benefit (income) expense	$ 534	$ 1,033	1,880	$ 3,097
Discount rate used to measure the plan obligations (as percent)					4.07%
Other comprehensive gain (loss) due to the results of fair value of plan assets			$ 19,700