Pension Plan and Postretirement Benefits Other Than Pension (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Amounts recognized in the statement of financial position:
Noncurrent liabilities	$ (48,810)	$ (45,936)
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
Change in projected benefit obligation:
Net benefit obligation at beginning of year	$ 208,085	172,105	$ 184,165
Service cost	12,080	10,084	11,011
Interest cost	8,420	8,395	7,711
Benefits paid	(10,184)	(8,733)	(19,283)
Actuarial (gain) loss	(7,618)	26,410	(11,499)
Plan amendments		(176)
Net benefit obligation at end of year	210,783	208,085	172,105
Accumulated benefit obligation	177,100	171,300
Change in plan assets:
Fair value of plan assets at beginning of year	175,548	170,430	133,911
Actual return on plan assets	(11,479)	(6,149)	38,802
Employer contributions	20,000	20,000	17,000
Benefits paid	(10,184)	(8,733)	(19,283)
Fair value of plan assets at end of year	173,885	175,548	170,430
Funded status at end of year	(36,898)	(32,537)	1,675
Amounts recognized in the statement of financial position:
Noncurrent liabilities	(36,898)	(32,537)	(1,675)
Net amount recognized at end of year	(36,898)	(32,537)	(1,675)
Amounts not yet reflected in net periodic benefit cost and included in accumulated other comprehensive income:
Transition obligation	(16)	(21)	(27)
Prior service credit (cost)	(720)	(1,179)	(1,822)
Accumulated gain (loss)	(88,882)	(75,681)	(30,602)
Accumulated other comprehensive income (loss)	(89,618)	(76,881)	(32,451)
Cumulative employer contributions in excess of (less than) net periodic benefit cost	52,720	44,344	30,776
Net amount recognized at end of year	$ (36,898)	$ (32,537)	$ (1,675)
Weighted average assumptions used to determine benefit obligation at December 31:
Discount rate (as a percent)	4.60%	4.13%	4.97%
Rate of compensation increase (as a percent)	5.12%	5.12%	5.12%
Other Postretirement Benefits
Pension Plan and Postretirement Benefits Other than Pension
Age of employees after which the plan does not provide benefits	65 years
Change in projected benefit obligation:
Net benefit obligation at beginning of year	$ 9,902	$ 8,172	$ 8,792
Service cost	910	719	788
Interest cost	397	397	361
Benefits paid	(505)	(527)	(283)
Actuarial (gain) loss	(2,632)	760	(1,807)
Retiree contributions	349	381	321
Net benefit obligation at end of year	8,421	9,902	8,172
Change in plan assets:
Employer contributions	156	146	(38)
Retiree contributions	349	381	321
Benefits paid	(505)	(527)	(283)
Funded status at end of year	(8,421)	(9,902)	(8,172)
Amounts recognized in the statement of financial position:
Current liabilities	(316)	(279)	(260)
Noncurrent liabilities	(8,105)	(9,623)	(7,912)
Net amount recognized at end of year	(8,421)	(9,902)	(8,172)
Amounts not yet reflected in net periodic benefit cost and included in accumulated other comprehensive income:
Prior service credit (cost)	2	(17)	(72)
Accumulated gain (loss)	2,897	265	1,041
Accumulated other comprehensive income (loss)	2,899	248	969
Cumulative employer contributions in excess of (less than) net periodic benefit cost	(11,320)	(10,150)	(9,141)
Net amount recognized at end of year	$ (8,421)	$ (9,902)	$ (8,172)
Weighted average assumptions used to determine benefit obligation at December 31:
Discount rate (as a percent)	4.44%	4.07%	4.94%