Employee Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Change in projected benefit obligation:
Settlement loss	$ 1,700	$ 4,100	$ 1,400	$ 900
Change in plan assets:
Fair value of plan assets at beginning of period				206,309	$ 206,309
Fair value of plan assets at end of period	156,678				156,678	$ 206,309
Amounts recognized on balance sheet:
Other current liabilities	(2,929)				(2,929)	(2,660)
Employee benefit obligation, noncurrent	(104,597)				(104,597)	(99,366)
Pension
Change in projected benefit obligation:
Obligation at beginning of period				248,505	248,505	220,330
Interest cost					8,073	8,844
Settlement loss					8,088
Actuarial (gain) loss					6,699	26,643
Benefits paid					(59,822)	(7,312)
Obligation at end of period	203,455				203,455	248,505	$ 220,330
Change in plan assets:
Fair value of plan assets at beginning of period				206,309	206,309	188,386
Actual return on plan assets					852	15,562
Employer contributions					9,339	9,673
Benefits paid					(59,822)	(7,312)
Fair value of plan assets at end of period	156,678				156,678	206,309	188,386
Funded status:
Plan assets less than projected benefit obligation	(46,777)				(46,777)	(42,196)
Amounts recognized on balance sheet:
Employee benefit obligation, noncurrent	(46,777)				(46,777)	(42,196)
Net amount recognized	(46,777)				(46,777)	(42,196)
Actuarial loss recognized in accumulated other comprehensive income (loss)	$ (37,945)				(37,945)	(27,728)
Actuarial assumptions:
Defined Benefit Plan, Net Periodic Benefit Cost					3,703	$ (3,685)	(2,888)
Benefits obligation settlement					55,900
Estimated future amount of loss to be amortized from accumulated other comprehensive income (loss)					$ 500
Pension | Minimum
Actuarial assumptions:
Discount rate used to measure the plan obligations (as percent)	4.09%				4.09%	3.70%
Pension | Maximum
Actuarial assumptions:
Discount rate used to measure the plan obligations (as percent)	4.10%				4.10%	3.74%
Other Postretirement Benefits
Change in projected benefit obligation:
Obligation at beginning of period				$ 59,830	$ 59,830	$ 50,988
Service cost					1,088	897	1,047
Interest cost					2,355	2,384
Actuarial (gain) loss					(765)	7,239
Benefits paid					(1,759)	(1,678)
Obligation at end of period	$ 60,749				60,749	59,830	50,988
Change in plan assets:
Employer contributions					1,759	1,678
Benefits paid					(1,759)	(1,678)
Funded status:
Plan assets less than projected benefit obligation	(60,749)				(60,749)	(59,830)
Amounts recognized on balance sheet:
Other current liabilities	(2,929)				(2,929)	(2,660)
Employee benefit obligation, noncurrent	(57,820)				(57,820)	(57,170)
Net amount recognized	(60,749)				(60,749)	(59,830)
Actuarial loss recognized in accumulated other comprehensive income (loss)	$ (9,577)				(9,577)	(10,931)
Actuarial assumptions:
Defined Benefit Plan, Net Periodic Benefit Cost					$ 4,031	$ 3,341	$ 3,492
Assumed health care cost trend rate, current (as a percent)					6.75%	7.00%	7.50%
Assumed health care cost trend rate, current (as a percent)					7.50%	6.75%
Assumed health care cost trend rate, ultimate (as a percent)					5.00%	5.00%	5.00%
Assumed health care cost trend rate, ultimate (as a percent)					5.00%	5.00%
Estimated future amount of loss to be amortized from accumulated other comprehensive income (loss)					$ 400
Other Postretirement Benefits | Minimum
Actuarial assumptions:
Discount rate used to measure the plan obligations (as percent)	4.32%				4.32%	3.97%
Other Postretirement Benefits | Maximum
Actuarial assumptions:
Discount rate used to measure the plan obligations (as percent)	4.56%				4.56%	4.15%