- Unrealized and Realized Gain/Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Natural gas commodity
Derivative [Line Items]
Amounts deferred to regulatory accounts on balance sheet	$ 2,548	$ 8,118	$ 19,895	$ (5,385)
Total gain (loss) in pre-tax earnings	10	(37)	(435)	(89)
Foreign exchange
Derivative [Line Items]
Amounts deferred to regulatory accounts on balance sheet	(51)	52	(275)	(5)
Total gain (loss) in pre-tax earnings	0	0	0	0
Benefit (expense) to cost of gas | Natural gas commodity
Derivative [Line Items]
Gain (loss) on derivative, net	(2,566)	(8,045)	(19,081)	5,562
Benefit (expense) to cost of gas | Foreign exchange
Derivative [Line Items]
Gain (loss) on derivative, net	51	(52)	275	5
Operating gain (loss) | Natural gas commodity
Derivative [Line Items]
Gain (loss) on derivative, net	28	(110)	1,249	266
Operating gain (loss) | Foreign exchange
Derivative [Line Items]
Gain (loss) on derivative, net	$ 0	$ 0	$ 0	$ 0