Employee Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Jan. 31, 2017
Pension and Other Postretirement Benefits
Total benefit (income) expense	$ (738)	$ (553)	$ (1,487)	$ (1,106)
Settlement loss	314		2,270
Pension
Pension and Other Postretirement Benefits
Interest cost	1,719	1,996	3,500	3,992
Expected asset return	(2,573)	(2,678)	(5,228)	(5,355)
Amortization of loss	116	129	241	257
Total benefit (income) expense	(424)	(553)	783	(1,106)
Other comprehensive gain due to the results of fair value of plan assets	800		800
Expected company contribution					$ 4,800
Expected company contribution current year			1,500
Pension | Union employees
Pension and Other Postretirement Benefits
Lump-sum benefits to plan participants in full settlement of obligations	$ 1,600		$ 12,200
Pension | Minimum
Pension and Other Postretirement Benefits
Discount rate used to measure the plan obligations (as percent)	3.61%		3.61%
Other Postretirement Benefits
Pension and Other Postretirement Benefits
Service cost	$ 215	259	$ 468	518
Interest cost	532	655	1,128	1,310
Amortization of loss	(213)	119	(251)	237
Total benefit (income) expense	$ 534	$ 1,033	1,345	$ 2,065
Discount rate used to measure the plan obligations (as percent)						4.07%
Other comprehensive gain due to the results of fair value of plan assets			$ 19,700