Employee Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Other Postretirement Benefits
Net periodic benefit (income) cost	$ (1,015)			$ (941)	$ (3,737)	$ (2,823)
Settlement loss	4,118				6,366
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	9,000				7,300
Expected company contribution current year	$ 9,300
Actuarial assumptions:
Discount rate (as a percent)	4.03%	4.09%	3.54%
Union employees
Other Postretirement Benefits
Lump-sum benefits to plan participants in full settlement of obligations	$ 25,700				45,500
Non-union employees
Other Postretirement Benefits
Lump-sum benefits to plan participants in full settlement of obligations					600
Pension
Other Postretirement Benefits
Interest cost	2,107			2,208	6,164	6,624
Expected asset return	(3,146)			(3,178)	(9,906)	(9,534)
Amortization of loss	24			29	5	87
Net benefit expense (income)	3,103			(941)	2,629	(2,823)
Expected company contribution							$ 10,000
Company contribution					7,600
Other Postretirement Benefits
Other Postretirement Benefits
Service cost	259			230	777	691
Interest cost	589			602	1,767	1,805
Amortization of loss	150			15	449	45
Net periodic benefit (income) cost	$ 998			$ 847	$ 2,993	$ 2,541