Defined Benefit Plan (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017	Dec. 31, 2017
Net Periodic Benefit Cost - Comprehensive Income Presentation [Abstract]
Pension non-service cost	$ 583		$ 871
Net periodic benefit cost	2,179		2,201
Components of Net Periodic Benefit Cost [Abstract]
Service cost	1,596		1,330
Interest cost	1,877		1,903
Other cost	1,133		1,038
Expected return on assets	(2,427)		(2,070)
Net periodic benefit cost	2,179		2,201
Plan Assets [Abstract]
Fair value of plan assets	137,981			$ 140,164
Employer Contributions [Abstract]
Capitalized pension non-service cost			128
Estimated employer contributions for the current fiscal year	7,450
Defined Contribution Plan, Employer Discretionary Contribution Amount	0
Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	87,205			88,990
Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	50,776			51,174
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	$ 5,928			8,207
Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	5,928			8,207
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	6,190	[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	6,151	[1]		6,376	[2]
Actively Managed | All Cap Equity | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	39	[1]		37	[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	50,495	[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	50,495	[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	[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	9,376	[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	9,376	[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	[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	9,306	[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	9,306	[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	[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	5,949	[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	5,949	[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	[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	5,864	[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	[2]
Actively Managed | REIT | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	5,864	[1]		6,143	[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	44,873	[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	[4]
Fixed Income | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	44,873	[3]		44,994	[4]
Fixed Income | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[3]		$ 0	[4]
ASU 2017-07
Net Periodic Benefit Cost - Comprehensive Income Presentation [Abstract]
Administrative and general	895		745
Maintenance expense	278		237
Pension non-service cost	583		871
Net periodic benefit cost	2,179		2,201
Components of Net Periodic Benefit Cost [Abstract]
Other cost	423		348
Net periodic benefit cost	$ 2,179		$ 2,201

[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.