Defined Benefit Plan (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019		Mar. 31, 2018	Dec. 31, 2018
Net Periodic Benefit Cost - Comprehensive Income Presentation [Abstract]
Other production expenses	$ 370		$ 423
Administrative and general	838		895
Maintenance expense	271		278
Pension non-service cost	921		583
Components of Net Periodic Benefit Cost [Abstract]
Service cost	1,479		1,596
Interest cost	2,112		1,877
Other cost	1,118		1,133
Expected return on assets	(2,309)		(2,427)
Net periodic benefit cost	2,400		$ 2,179
Plan Assets [Abstract]
Fair value of plan assets	146,822			$ 133,459
Employer Contributions [Abstract]
Estimated employer contributions for the current fiscal year	8,337
Defined Contribution Plan, Employer Discretionary Contribution Amount	0
Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	93,543			82,677
Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	53,279			50,782
Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	$ 0			0
Equity Securities [Member]
Plan Assets [Abstract]
Target plan asset allocations	55.00%
Fixed Income Securities
Plan Assets [Abstract]
Target plan asset allocations	45.00%
Cash and cash equivalents
Plan Assets [Abstract]
Fair value of plan assets	$ 7,309			8,136
Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	7,309			8,136
Cash and cash equivalents | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0			0
Cash and cash equivalents | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0			0
Actively Managed | All Cap Equity
Plan Assets [Abstract]
Fair value of plan assets	6,373	[1]		5,670	[2]
Actively Managed | All Cap Equity | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	6,331	[1]		5,632	[2]
Actively Managed | All Cap Equity | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	42	[1]		38	[2]
Actively Managed | All Cap Equity | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[2]
Actively Managed | U.S. Large Cap Equity
Plan Assets [Abstract]
Fair value of plan assets	54,541	[1]		47,040	[2]
Actively Managed | U.S. Large Cap Equity | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	54,541	[1]		47,040	[2]
Actively Managed | U.S. Large Cap Equity | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[2]
Actively Managed | U.S. Large Cap Equity | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[2]
Actively Managed | U.S. Mid Cap Equity
Plan Assets [Abstract]
Fair value of plan assets	9,673	[1]		8,372	[2]
Actively Managed | U.S. Mid Cap Equity | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	9,673	[1]		8,372	[2]
Actively Managed | U.S. Mid Cap Equity | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[2]
Actively Managed | U.S. Mid Cap Equity | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[2]
Actively Managed | U.S. Small Cap Equity
Plan Assets [Abstract]
Fair value of plan assets	10,187	[1]		8,528	[2]
Actively Managed | U.S. Small Cap Equity | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	10,187	[1]		8,528	[2]
Actively Managed | U.S. Small Cap Equity | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[2]
Actively Managed | U.S. Small Cap Equity | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[2]
Actively Managed | Non-U.S. Large Cap Equity
Plan Assets [Abstract]
Fair value of plan assets	5,502	[1]		4,969	[2]
Actively Managed | Non-U.S. Large Cap Equity | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	5,502	[1]		4,969	[2]
Actively Managed | Non-U.S. Large Cap Equity | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[2]
Actively Managed | Non-U.S. Large Cap Equity | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[2]
Actively Managed | REIT
Plan Assets [Abstract]
Fair value of plan assets	6,885	[1]		5,889	[2]
Actively Managed | REIT | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[2]
Actively Managed | REIT | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	6,885	[1]		5,889	[2]
Actively Managed | REIT | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[2]
Fixed Income
Plan Assets [Abstract]
Fair value of plan assets	46,352	[3]		44,855	[4]
Fixed Income | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	0	[3]		0	[4]
Fixed Income | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	46,352	[3]		44,855	[4]
Fixed Income | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	$ 0	[3]		$ 0	[4]

[1] Actively managed portfolio of securities with the goal to exceed the stated benchmark performance.

[2] Actively managed portfolio of securities with the goal to exceed the stated benchmark performance.

[3] Actively managed portfolio of fixed income securities with the goal to exceed the Barclays 1-5 Year Government/Credit, Barclays Intermediate Government/Credit, and Merrill Lynch Preferred Stock Fixed Rate.

[4] Actively managed portfolio of fixed income securities with the goal to exceed the Barclays 1-5 Year Government/Credit, Barclays Intermediate Government/Credit, and Merrill Lynch Preferred Stock Fixed Rate.