Defined Benefit Plan (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2019		Sep. 30, 2018	Sep. 30, 2019		Sep. 30, 2018	Dec. 31, 2018
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 1,479		$ 1,601	$ 4,437		$ 4,804
Interest cost	2,113		1,876	6,338		5,629
Other cost	1,117		1,139	3,352		3,417
Expected return on assets	(2,310)		(2,426)	(6,929)		(7,279)
Net periodic benefit cost	2,399		2,190	7,198		6,571
Plan Assets [Abstract]
Fair value of plan assets	154,992			154,992			$ 133,459
Employer Contributions [Abstract]
Estimated employer contributions for the current fiscal year	8,337			8,337
Defined Contribution Plan, Employer Discretionary Contribution Amount	3,854
Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	93,882			93,882			82,677
Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	61,110			61,110			50,782
Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	$ 0			$ 0			0
Equity Securities [Member]
Plan Assets [Abstract]
Target plan asset allocations	55.00%			55.00%
Fixed Income Securities
Plan Assets [Abstract]
Target plan asset allocations	45.00%			45.00%
Cash and cash equivalents
Plan Assets [Abstract]
Fair value of plan assets	$ 7,884			$ 7,884			8,136
Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	7,884			7,884			8,136
Cash and cash equivalents | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0			0			0
Cash and cash equivalents | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0			0			0
Actively Managed | All Cap Equity
Plan Assets [Abstract]
Fair value of plan assets	6,667	[1]		6,667	[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	6,667	[1]		6,667	[1]		5,632	[2]
Actively Managed | All Cap Equity | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]		38	[2]
Actively Managed | All Cap Equity | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]		0	[2]
Actively Managed | U.S. Large Cap Equity
Plan Assets [Abstract]
Fair value of plan assets	53,141	[1]		53,141	[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	53,141	[1]		53,141	[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	[1]		0	[2]
Actively Managed | U.S. Large Cap Equity | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]		0	[2]
Actively Managed | U.S. Mid Cap Equity
Plan Assets [Abstract]
Fair value of plan assets	10,179	[1]		10,179	[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	10,179	[1]		10,179	[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	[1]		0	[2]
Actively Managed | U.S. Mid Cap Equity | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]		0	[2]
Actively Managed | U.S. Small Cap Equity
Plan Assets [Abstract]
Fair value of plan assets	10,373	[1]		10,373	[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	10,373	[1]		10,373	[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	[1]		0	[2]
Actively Managed | U.S. Small Cap Equity | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]		0	[2]
Actively Managed | Non-U.S. Large Cap Equity
Plan Assets [Abstract]
Fair value of plan assets	5,638	[1]		5,638	[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	5,638	[1]		5,638	[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	[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	[1]		0	[2]
Actively Managed | REIT
Plan Assets [Abstract]
Fair value of plan assets	7,779	[1]		7,779	[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	[1]		0	[2]
Actively Managed | REIT | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	7,779	[1]		7,779	[1]		5,889	[2]
Actively Managed | REIT | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]		0	[2]
Fixed Income
Plan Assets [Abstract]
Fair value of plan assets	53,331	[3]		53,331	[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	[3]		0	[4]
Fixed Income | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	53,331	[3]		53,331	[3]		44,855	[4]
Fixed Income | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[3]		0	[3]		$ 0	[4]
Other production expenses
Components of Net Periodic Benefit Cost [Abstract]
Net periodic benefit cost	370		424	1,110		1,273
General and Administrative Expense
Components of Net Periodic Benefit Cost [Abstract]
Net periodic benefit cost	837		898	2,526		2,695
Maintenance expense
Components of Net Periodic Benefit Cost [Abstract]
Net periodic benefit cost	271		279	813		836
Pension non-service costs
Components of Net Periodic Benefit Cost [Abstract]
Net periodic benefit cost	$ 921		$ 589	$ 2,749		$ 1,767

[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.