Lease Commitments - Components of Lease Expense (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Lease, Cost [Line Items]
Total operating lease expense	$ 7,611
Financing lease expense:
Amortization of right-of-use assets	125
Interest on lease liabilities	32
Total financing lease expense	157
Variable lease expense	0
Short-term lease expense	94
Total lease expense for the period	7,862
PNM
Lease, Cost [Line Items]
Total operating lease expense	7,231
Financing lease expense:
Amortization of right-of-use assets	66
Interest on lease liabilities	16
Total financing lease expense	82
Variable lease expense	0
Short-term lease expense	74
Total lease expense for the period	7,387
TNMP
Lease, Cost [Line Items]
Total operating lease expense	241
Financing lease expense:
Amortization of right-of-use assets	58
Interest on lease liabilities	17
Total financing lease expense	75
Variable lease expense	0
Short-term lease expense	3
Total lease expense for the period	$ 319