Interest Rate Swap Agreement (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Interest Rate Swap Agreement [Abstract]
Long-term debt	$ 91,701		$ 91,701		$ 87,488
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		$ 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		$ 12,000
Fixed interest rate	3.16%		3.16%
Net payment rate on swap	2.43%	2.94%	2.53%	2.87%
Interest rate swap settlements reclassified from regulatory assets to interest expense	$ 74	$ 86	$ 229	$ 259
Interest rate swap loss capitalized	(44)	$ (25)	(173)	$ (868)
Interest rate swap settlements to be reclassified during the next 12 months	274		274
Potential payment to counterparty	$ 2,367		$ 2,367
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%		59.00%
Term of variable rate			1 month