Concentrations of Credit Risk (Details) - Customer Concentration Risk [Member]	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Sales Revenue, Goods, Net [Member] | Duke Energy [Member]
Concentration Risk [Line Items]
Concentration Risk, Customer		242020
Sales Revenue, Goods, Net [Member] | NJR Energy Services Company [Member]
Concentration Risk [Line Items]
Concentration Risk, Customer			295779
Accounts Receivable [Member]
Concentration Risk [Line Items]
Concentration Risk, Customer	220222	151383
Accounts Receivable [Member] | Gas Customers [Member]
Concentration Risk [Line Items]
Concentration Risk, Customer	124509	72664
Accounts Receivable [Member] | All Other [Member]
Concentration Risk [Line Items]
Concentration Risk, Customer	4233	6003
Accounts Receivable [Member] | Coal Brokers and Distributors [Member]
Concentration Risk [Line Items]
Concentration Risk, Customer	28955	14435
Accounts Receivable [Member] | Steam Coal Utilities [Member]
Concentration Risk [Line Items]
Concentration Risk, Customer	62525	58281