Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating revenues	$ 264,712	$ 297,323
Depreciation and amortization	20,985	21,085
Income (loss) from operations	67,348	77,532
Net Income	41,537	40,310
Capital expenditures	57,431	38,924
Total Assets	3,029,339	3,070,656	$ 3,039,746
Utility
Segment Reporting Information [Line Items]
Operating revenues	257,933
Gas Storage
Segment Reporting Information [Line Items]
Operating revenues	5,233
Other
Segment Reporting Information [Line Items]
Operating revenues	1,546
Operating Segments | Utility
Segment Reporting Information [Line Items]
Operating revenues	257,933	292,726
Depreciation and amortization	20,543	19,624
Income (loss) from operations	64,756	77,127
Net Income	39,883	40,192
Capital expenditures	56,894	38,854
Total Assets	2,951,918	2,799,638	2,961,326
Operating Segments | Gas Storage
Segment Reporting Information [Line Items]
Operating revenues	5,233	4,541
Depreciation and amortization	442	1,461
Income (loss) from operations	3,036	606
Net Income	1,898	61
Capital expenditures	537	70
Total Assets	58,676	254,260	59,583
Operating Segments | Other
Segment Reporting Information [Line Items]
Operating revenues	1,546	56
Depreciation and amortization	0	0
Income (loss) from operations	(444)	(201)
Net Income	(244)	57
Capital expenditures	0	0
Total Assets	$ 18,745	$ 16,758	$ 18,837