Segment Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) segment	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Operating revenues	$ 88,190	$ 87,727	$ 521,751	$ 442,439
Depreciation and amortization	21,484	20,628	63,924	61,435
Income (loss) from operations	(6,656)	(4,437)	82,729	82,241
Net income	(8,495)	(8,040)	34,544	30,620
Capital expenditures	51,123	36,675	145,441	98,111
Total Assets	3,105,607	2,959,884	$ 3,105,607	2,959,884	$ 3,079,801
Operating Segments
Segment Reporting Information [Line Items]
Number of reportable segments | segment			2
Operating Segments | Utility
Segment Reporting Information [Line Items]
Operating revenues	81,126	80,378	$ 503,947	422,617
Depreciation and amortization	20,023	19,173	59,541	56,894
Income (loss) from operations	(9,977)	(7,264)	77,706	74,745
Net income	(10,349)	(9,511)	31,980	26,848
Capital expenditures	50,009	36,238	143,128	96,710
Total Assets	2,835,860	2,684,618	2,835,860	2,684,618	2,806,627
Operating Segments | Gas Storage
Segment Reporting Information [Line Items]
Operating revenues	7,006	7,293	17,635	19,654
Depreciation and amortization	1,461	1,455	4,383	4,541
Income (loss) from operations	3,543	3,502	5,748	8,107
Net income	1,899	1,813	2,716	3,988
Capital expenditures	164	437	1,363	1,401
Total Assets	252,041	259,483	252,041	259,483	256,333
Operating Segments | Other
Segment Reporting Information [Line Items]
Operating revenues	58	56	169	168
Depreciation and amortization	0	0	0	0
Income (loss) from operations	(222)	(675)	(725)	(611)
Net income	(45)	(342)	(152)	(216)
Capital expenditures	950	0	950	0
Total Assets	$ 17,706	$ 15,783	$ 17,706	$ 15,783	$ 16,841