Defined Benefit Pension Plans Multiemployer Pension Plans (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Multiemployer Plans [Line Items]
Multiemployer plan, period contributions	$ 910	$ 887	$ 802
National Electric Benefit Fund [Member]
Multiemployer Plans [Line Items]
Number of collective-bargaining agreements	10
Multiemployer plan, period contributions	$ 768	785	712
Eighth District Electrical Pension Fund [Member]
Multiemployer Plans [Line Items]
Number of collective-bargaining agreements	1
Multiemployer plan, period contributions	$ 142	$ 102	$ 90