Segment Information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Operating revenues	$ 136,238	$ 99,183	$ 433,561	$ 354,712
Depreciation and amortization	21,355	20,413	42,440	40,807
Income (loss) from operations	13,157	12,606	89,385	86,678
Net income	2,729	2,019	43,039	38,660
Capital expenditures	55,394	32,099	94,318	62,153
Total Assets	3,065,154	2,943,144	$ 3,065,154	2,943,144	$ 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	130,095	92,135	$ 422,821	342,239
Depreciation and amortization	19,894	18,961	39,518	37,721
Income (loss) from operations	11,860	9,714	87,683	82,009
Net income	2,137	507	42,329	36,359
Capital expenditures	54,265	31,295	93,119	60,472
Total Assets	2,792,011	2,663,817	2,792,011	2,663,817	2,806,627
Operating Segments | Gas Storage
Segment Reporting Information [Line Items]
Operating revenues	6,088	6,992	10,629	12,361
Depreciation and amortization	1,461	1,452	2,922	3,086
Income (loss) from operations	1,599	2,879	2,205	4,605
Net income	756	1,439	817	2,175
Capital expenditures	1,129	804	1,199	1,681
Total Assets	256,396	263,498	256,396	263,498	256,333
Operating Segments | Other
Segment Reporting Information [Line Items]
Operating revenues	55	56	111	112
Depreciation and amortization	0	0	0	0
Income (loss) from operations	(302)	13	(503)	64
Net income	(164)	73	(107)	126
Capital expenditures	0	0	0	0
Total Assets	$ 16,747	$ 15,829	$ 16,747	$ 15,829	$ 16,841