Defined Benefit Pension Plans Multiemployer Pension Plans (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Multiemployer Plans [Line Items]
Multiemployer plan, period contributions	$ 802	$ 893	$ 848
National Electric Benefit Fund [Member]
Multiemployer Plans [Line Items]
Number of collective-bargaining agreements	9
Multiemployer plan, period contributions	$ 712	774	743
Eighth District Electrical Pension Fund [Member]
Multiemployer Plans [Line Items]
Number of collective-bargaining agreements	1
Multiemployer plan, period contributions	$ 90	$ 119	$ 105