Unrealized and Realized Gain/Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Natural gas commodity
Derivative [Line Items]
Amounts deferred to regulatory accounts on balance sheet	$ 5,895	$ 13,893
Total gain (loss) in pre-tax earnings	(79)	(427)
Foreign exchange
Derivative [Line Items]
Amounts deferred to regulatory accounts on balance sheet	154	(26)
Total gain (loss) in pre-tax earnings	0	0
Benefit (expense) to cost of gas | Natural gas commodity
Derivative [Line Items]
Gain (loss) on derivative, net	(5,747)	(13,094)
Benefit (expense) to cost of gas | Foreign exchange
Derivative [Line Items]
Gain (loss) on derivative, net	(154)	26
Operating Expense [Member] | Natural gas commodity
Derivative [Line Items]
Gain (loss) on derivative, net	(227)	(1,226)
Operating Expense [Member] | Foreign exchange
Derivative [Line Items]
Gain (loss) on derivative, net	$ 0	$ 0