Defined Benefit Plan (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017	Dec. 31, 2017
Net Periodic Benefit Cost - Comprehensive Income Presentation [Abstract]
Pension non-service cost	$ 589		$ 953	$ 1,767		$ 2,860
Capitalized pension non-service cost			420			420
Components of Net Periodic Benefit Cost [Abstract]
Service cost	1,601		1,308	4,804		3,922
Interest cost	1,876		1,912	5,629		5,735
Other cost	1,139		1,101	3,417		3,304
Expected return on assets	(2,426)		(2,060)	(7,279)		(6,179)
Net periodic benefit cost	2,190		2,261	6,571		6,782
Plan Assets [Abstract]
Fair value of plan assets	145,031			145,031			$ 140,164
Employer Contributions [Abstract]
Estimated employer contributions for the current fiscal year	7,450			7,450
Defined Contribution Plan, Employer Discretionary Contribution Amount	2,048			3,468
Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	92,116			92,116			88,990
Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	52,915			52,915			51,174
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	$ 4,286			$ 4,286			8,207
Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	4,286			4,286			8,207
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	6,613	[1]		6,613	[1]		6,413	[2]
Actively Managed | All Cap Equity | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	6,572	[1]		6,572	[1]		6,376	[2]
Actively Managed | All Cap Equity | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	41	[1]		41	[1]		37	[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	55,782	[1]		55,782	[1]		50,351	[2]
Actively Managed | U.S. Large Cap Equity | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	55,782	[1]		55,782	[1]		50,351	[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	9,747	[1]		9,747	[1]		9,358	[2]
Actively Managed | U.S. Mid Cap Equity | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	9,747	[1]		9,747	[1]		9,358	[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	9,848	[1]		9,848	[1]		8,725	[2]
Actively Managed | U.S. Small Cap Equity | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	9,848	[1]		9,848	[1]		8,725	[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	5,881	[1]		5,881	[1]		5,973	[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	5,881	[1]		5,881	[1]		5,973	[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	6,346	[1]		6,346	[1]		6,143	[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	6,346	[1]		6,346	[1]		6,143	[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	46,528	[3]		46,528	[3]		44,994	[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	46,528	[3]		46,528	[3]		44,994	[4]
Fixed Income | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[3]		0	[3]		$ 0	[4]
ASU 2017-07
Net Periodic Benefit Cost - Comprehensive Income Presentation [Abstract]
Administrative and general	898		733	2,695		2,197
Maintenance expense	279		233	836		698
Pension non-service cost	589		953	1,767		2,860
Components of Net Periodic Benefit Cost [Abstract]
Other cost	424		342	1,273		1,027
Net periodic benefit cost	$ 2,190		$ 2,261	$ 6,571		$ 6,782

[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.