Agreement and Plan of Merger Agreement and Plan of Merger (Details) $ / shares in Units, $ in Thousands				3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) $ / shares	Mar. 06, 2020	Jan. 29, 2020	Mar. 31, 2020 USD ($) $ / shares	Mar. 31, 2019 USD ($)	Mar. 31, 2020 USD ($) $ / shares	Mar. 31, 2019 USD ($)
Public Utilities, General Disclosures [Line Items]
Business Acquisition, Share Price | $ / shares	$ 68.25			$ 68.25		$ 68.25
Contract Extension Period				3 months
Loss Contingency, Number of Plaintiffs	3			3
Number of Installments				36
Business Combination, Acquisition Related Costs				$ 1,295	$ 0	$ 13,405	$ 0
Loss Contingency, Claims Dismissed, Number		1	1
FERC
Public Utilities, General Disclosures [Line Items]
Period to file proposed mitigation with regulatory agency				45 days
Maximum | FERC
Public Utilities, General Disclosures [Line Items]
Period to file proposed mitigation with regulatory agency				45 days
Maximum | PUCT
Public Utilities, General Disclosures [Line Items]
Period to file tariff with regulatory agency				45 days
Maximum | NMPRC
Public Utilities, General Disclosures [Line Items]
Period to file tariff with regulatory agency				7 days
El Paso Electric Company
Public Utilities, General Disclosures [Line Items]
Business Combination, Merger Agreement, Potential Termination Fee	$ 85,000			$ 85,000		85,000
Sun Jupiter Holdings LLC
Public Utilities, General Disclosures [Line Items]
Business Combination, Merger Agreement, Potential Termination Fee	170,000			$ 170,000		170,000
U.S. District Court for the Southern District of New York
Public Utilities, General Disclosures [Line Items]
Loss Contingency, Number of Plaintiffs				2
U.S. District Court for the District of Delaware
Public Utilities, General Disclosures [Line Items]
Loss Contingency, Number of Plaintiffs				1
Texas Customers
Public Utilities, General Disclosures [Line Items]
Potential Rate Credits to Customers	21,000			$ 21,000		21,000
New Mexico Customers
Public Utilities, General Disclosures [Line Items]
Potential Rate Credits to Customers	$ 8,700			$ 8,700		$ 8,700