Commitments and Contingencies - Letters of Credit and Guarantees (Detail)	Mar. 31, 2016 USD ($)
Guarantee Obligations [Line Items]
Year of expiration 2016	$ 89,400,000
Year of expiration 2017-2020	0
Year of expiration After 2020	92,900,000
Maximum Theoretical Obligation	182,300,000
Liabilities Recognized at Mar 31, 2016	0
Letter of Credit [Member]
Guarantee Obligations [Line Items]
Year of expiration 2016	0
Year of expiration 2017-2020	0
Year of expiration After 2020	2,200,000
Maximum Theoretical Obligation	2,200,000
Liabilities Recognized at Mar 31, 2016	100,000
Tampa Electric Company [Member] | Letters Of Credit [Member]
Guarantee Obligations [Line Items]
Year of expiration 2016	0
Year of expiration 2017-2020	0
Year of expiration After 2020	500,000
Maximum Theoretical Obligation	500,000
Liabilities Recognized at Mar 31, 2016	100,000
New Mexico Gas Company [Member] | Letters Of Credit [Member]
Guarantee Obligations [Line Items]
Year of expiration 2016	0
Year of expiration 2017-2020	0
Year of expiration After 2020	1,700,000
Maximum Theoretical Obligation	1,700,000
Liabilities Recognized at Mar 31, 2016	0
Fuel Sales and Transportation [Member]
Guarantee Obligations [Line Items]
Year of expiration 2016	0
Year of expiration 2017-2020	0
Year of expiration After 2020	92,900,000
Maximum Theoretical Obligation	92,900,000
Liabilities Recognized at Mar 31, 2016	0
Letters of Indemnity - Coal Mining Permits
Guarantee Obligations [Line Items]
Year of expiration 2016	89,400,000
Year of expiration 2017-2020	0
Year of expiration After 2020	0
Maximum Theoretical Obligation	89,400,000
Liabilities Recognized at Mar 31, 2016	$ 0