Employee Benefit Plans (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule Of Employee Benefit Plans Disclosures [Line Items]
Employee benefit plan expense	$ 1,770	$ 8,471	$ 19,821
Defined Contribution Retirement Plans
Number of plans | Item	2
Employer's matching contribution limit of eligible compensation	8.00%
Contributions	$ 7,241	10,312	11,725
Retiree Medical Plan
Increase (Decrease) in Postretirement Obligations	794	1,792	3,874
Postretirement Medical Plan
Service cost	894	2,225	4,916
Interest cost	918	1,187	1,929
Amortization of prior service cost	(7,283)	(5,253)	1,252
Net periodic benefit cost	(5,471)	(1,841)	8,096
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Balance at the beginning of the period	23,562	61,407	50,697
Current period service costs	894	2,225	4,916
Interest cost	918	1,187	1,929
Defined Benefit Plan, Plan Amendments	0	(47,671)	0
Benefits paid, net of retiree contributions	(277)	(198)	(9)
Change in actuarial assumptions and actuarial loss	794	6,612	3,874
Balance at the end of the period	25,891	23,562	61,407
Less current portion	933	612	562
Long-term APBO	24,958	22,950	$ 60,845
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	$ 13,807	$ 21,884
Assumptions in the measurement of the APBO
Discount rate (as a percent)	3.46%	3.95%	4.14%
Health care cost trend rate assumed for next year (as a percent)			7.50%
Ultimate health care cost trend rate (as a percent)			5.00%
Year that the rate reaches the ultimate trend rate			2026
Estimated future benefit payments:
2018	$ 949
2019	896
2020	1,062
2021	1,204
2022	1,324
2023 - 2027	7,832
Defined Contribution Retirement Plan [Member]
Schedule Of Employee Benefit Plans Disclosures [Line Items]
Employee benefit plan expense	$ 7,241	$ 10,312	$ 11,725
Retiree Medical Plan
Age of employees, entitled to receive benefits under the Retiree Medical Plan (in years)	55 years
Minimum completed term of service, employees entitled to receive benefits under the Retiree Medical Plan (in years)	10 years
Increase (Decrease) in Postretirement Obligations	$ 47,700
Negative prior service cost base to be amortized	$ (41,100)
Postretirement Credit Amortization Period	4 years 2 months
Post retirement medical plan
Schedule Of Employee Benefit Plans Disclosures [Line Items]
Employee benefit plan expense	$ (5,471)	(1,841)	$ 8,096
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net prior service credit	24,062	33,804
Net actuarial loss	(16,520)	(18,184)
Accumulated other comprehensive income (loss) before tax	7,542	15,620
Income taxes	611	611
Accumulated other comprehensive loss	8,153	$ 16,231
DefinedBenefitPlanAmountsThatWillBeAmortizedFromAccumulatedOtherComprehensiveIncomeLossInNextFiscalYearAbstract
Net Prior Service Credit	(9,742)
Net actuarial loss (gain)	(2,392)
Total amortized next fiscal year	$ (7,350)