Commitments and Contingencies - Letters of Credit and Guarantees (Detail) (USD $)	Mar. 31, 2015
Tampa Electric Company [Member] | Letters Of Credit [Member]
Guarantee Obligations [Line Items]
Year of Expiration	$ 0
Year of expiration 2016-2019	0
Year of expiration After 2019	600,000
Maximum Theoretical Obligation	600,000
Liabilities Recognized at Mar 31, 2015	100,000
New Mexico Gas Company [Member] | Letters Of Credit [Member]
Guarantee Obligations [Line Items]
Year of Expiration	0
Year of expiration 2016-2019	0
Year of expiration After 2019	2,300,000
Maximum Theoretical Obligation	2,300,000
Liabilities Recognized at Mar 31, 2015	0
Fuel Sales and Transportation [Member]
Guarantee Obligations [Line Items]
Year of Expiration	0
Year of expiration 2016-2019	0
Year of expiration After 2019	92,900,000
Maximum Theoretical Obligation	92,900,000
Liabilities Recognized at Mar 31, 2015	$ 0