Long-Term Debt and Short-Term Borrowings, Variable Rate Long-Term Debt (Details) - Variable Rate Pennsylvania Economic Development Financing Authority Exempt Facilities Revenue Bonds Series 2008A due 2029 [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Variable Rate Long-Term Debt [Abstract]
Face value	$ 12,000
Maturity date	Oct. 01, 2029
Annual average variable interest rate	0.88%	0.47%	0.06%
Variable interest rate at year end	1.78%	0.80%
Period in which to reimburse bank for purchase price of tendered bonds that have not been remarketed	14 months
Letter of Credit [Member]
Variable Rate Long-Term Debt [Abstract]
Expiration date	Jun. 30, 2019