Employee Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Pension and Other Postretirement Benefits
Net periodic benefit cost (income)	$ (553)	$ (1,343)		$ (1,106)	$ (2,721)
Settlement loss		1,397			2,248
Discount rate used to measure the plan obligations (as percent)			3.57%
Other comprehensive income due to the results of fair value of plan assets		3,900			$ 1,700
Actuarial assumptions:
Discount rate (as a percent)			3.54%		4.09%
Union employees
Pension and Other Postretirement Benefits
Lump-sum benefits to plan participants in full settlement of obligations		13,700
Non-union employees
Pension and Other Postretirement Benefits
Lump-sum benefits to plan participants in full settlement of obligations					$ 600
Pension
Pension and Other Postretirement Benefits
Interest cost	1,996	1,984		3,992	4,057
Expected asset return	(2,678)	(3,366)		(5,355)	(6,760)
Amortization of (gain) loss	129	39		257	(18)
Total benefit income	(553)	54		(1,106)	(473)
Expected company contribution						$ 9,300
Company contribution				3,900
Pension | Union employees
Pension and Other Postretirement Benefits
Lump-sum benefits to plan participants in full settlement of obligations					19,700
Other Postretirement Benefits
Pension and Other Postretirement Benefits
Service cost	259	259		518	518
Interest cost	655	589		1,310	1,178
Amortization of (gain) loss	119	149		237	299
Net periodic benefit cost (income)	$ 1,033	$ 997		$ 2,065	$ 1,995