Long-term debt Issued (Details 2) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 13, 2019	Jun. 07, 2019	Apr. 04, 2019	Dec. 31, 2018	Apr. 25, 2017
Long-term debt issued [Line Items]
Long-term debt	$ 2,246,756				$ 2,108,695
Senior Notes
Long-term debt issued [Line Items]
Long-term debt	$ 1,655,000				1,381,000
Weighted Average Interest Rate	4.52%
Credit Agreements
Long-term debt issued [Line Items]
Long-term debt	$ 31,000				$ 110,100
Weighted Average Interest Rate	4.66%
Centennial Energy Holdings, Inc. [Member] | Senior Notes
Long-term debt issued [Line Items]
Long-term debt				$ 150,000
Weighted Average Interest Rate				4.60%
Cascade Natural Gas [Member] | Senior Notes
Long-term debt issued [Line Items]
Long-term debt		$ 75,000
Weighted Average Interest Rate		3.93%
Cascade Natural Gas [Member] | Credit Agreements
Long-term debt issued [Line Items]
Long-term debt			$ 100,000			$ 75,000
Intermountain Gas Company [Member] | Senior Notes
Long-term debt issued [Line Items]
Long-term debt		$ 50,000
Weighted Average Interest Rate		3.92%