Interest Rate Swap Agreement (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Interest Rate Swap Agreement [Abstract]
Long-term debt	$ 91,692		$ 92,833
Variable Rate Pennsylvania Economic Development Financing Authority Exempt Facilities Revenue Refunding Bonds, Series 2008A, due 2029 [Member]
Interest Rate Swap Agreement [Abstract]
Long-term debt	12,000		$ 12,000
Variable Rate Pennsylvania Economic Development Financing Authority Exempt Facilities Revenue Refunding Bonds, Series 2008A, due 2029 [Member] | Interest Rate Swap [Member]
Interest Rate Swap Agreement [Abstract]
Notional amount of swap	$ 12,000
Fixed interest rate	3.16%
Net payment rate on swap	2.18%	2.64%
Interest rate swap settlements reclassified from regulatory assets to interest expense	$ 65	$ 79
Interest rate swap (gain) loss capitalized	(257)	$ 9
Interest rate swap settlements to be reclassified during the next 12 months	225
Potential payment to counterparty	$ 1,907
Variable Rate Pennsylvania Economic Development Financing Authority Exempt Facilities Revenue Refunding Bonds, Series 2008A, due 2029 [Member] | LIBOR [Member] | Interest Rate Swap [Member]
Interest Rate Swap Agreement [Abstract]
Percentage of variable interest rate	59.00%
Term of variable rate	1 month