Pension Plan and Postretirement Benefits Other Than Pension - Changes in net funded status, disclosure of amounts recognized in the balance sheet, and the assumptions used to determine the benefit (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amounts recognized in the statement of financial position:
Noncurrent liabilities	$ (38,379)		$ (38,379)	$ (48,810)
Pension Benefits
Pension Plan and Postretirement Benefits Other than Pension
Final number of years of employee's compensation to determine the benefits payable			10 years
Non-cash settlement charge	(20,700)		$ (20,681)
Change in projected benefit obligation:
Net benefit obligation at beginning of year			210,783	208,085	$ 172,105
Service cost			12,199	12,080	10,084
Interest cost			9,432	8,420	8,395
Benefits paid			(52,288)	(10,184)	(8,733)
Actuarial (gain) loss			795	(7,618)	26,410
Plan amendments					(176)
Net benefit obligation at end of year	180,921		180,921	210,783	208,085
Accumulated benefit obligation	150,100		150,100	177,100
Change in plan assets:
Fair value of plan assets at beginning of year			173,885	175,548	170,430
Actual return on plan assets			2,932	(11,479)	(6,149)
Employer contributions			20,000	20,000	20,000
Benefits paid			(52,288)	(10,184)	(8,733)
Fair value of plan assets at end of year	144,529		144,529	173,885	175,548
Funded status at end of year	(36,392)		(36,392)	(36,898)	(32,537)
Amounts recognized in the statement of financial position:
Noncurrent liabilities	(36,392)		(36,392)	(36,898)	(32,537)
Net amount recognized at end of year	(36,392)		(36,392)	(36,898)	(32,537)
Amounts not yet reflected in net periodic benefit cost and included in accumulated other comprehensive income:
Transition obligation	(11)		(11)	(16)	(21)
Prior service credit (cost)	(346)		(346)	(720)	(1,179)
Accumulated gain (loss)	(73,775)		(73,775)	(88,882)	(75,681)
Accumulated other comprehensive income (loss)	(74,132)		(74,132)	(89,618)	(76,881)
Cumulative employer contributions in excess of (less than) net periodic benefit cost	37,740		37,740	52,720	44,344
Net amount recognized at end of year	$ (36,392)		$ (36,392)	$ (36,898)	$ (32,537)
Weighted average assumptions used to determine benefit obligation at December 31:
Discount rate (as a percent)	4.39%		4.39%	4.60%	4.13%
Rate of compensation increase (as a percent)	5.12%		5.12%	5.12%	5.12%
Other Postretirement Benefits
Pension Plan and Postretirement Benefits Other than Pension
Curtailment gain		$ 8,500	$ 8,475
Change in projected benefit obligation:
Net benefit obligation at beginning of year			8,421	$ 9,902	$ 8,172
Service cost			555	910	719
Interest cost			297	397	397
Benefits paid			(674)	(505)	(527)
Actuarial (gain) loss			1,790	(2,632)	760
Retiree contributions			532	349	381
Curtailment gain			(8,475)
Net benefit obligation at end of year	$ 2,446		2,446	8,421	9,902
Change in plan assets:
Employer contributions			142	156	146
Retiree contributions			532	349	381
Benefits paid			(674)	(505)	(527)
Funded status at end of year	(2,446)		(2,446)	(8,421)	(9,902)
Amounts recognized in the statement of financial position:
Current liabilities	(458)		(458)	(316)	(279)
Noncurrent liabilities	(1,988)		(1,988)	(8,105)	(9,623)
Net amount recognized at end of year	(2,446)		(2,446)	(8,421)	(9,902)
Amounts not yet reflected in net periodic benefit cost and included in accumulated other comprehensive income:
Prior service credit (cost)	6		6	2	(17)
Accumulated gain (loss)	954		954	2,897	265
Accumulated other comprehensive income (loss)	960		960	2,899	248
Cumulative employer contributions in excess of (less than) net periodic benefit cost	(3,406)		(3,406)	(11,320)	(10,150)
Net amount recognized at end of year	$ (2,446)		$ (2,446)	$ (8,421)	$ (9,902)
Weighted average assumptions used to determine benefit obligation at December 31:
Discount rate (as a percent)	3.46%		3.46%	4.44%	4.07%