Short-Term Debt - Credit Facilities (Detail) - USD ($)	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Sep. 02, 2014	Dec. 17, 2013
Line Of Credit Facility [Line Items]
Credit Facilities	$ 900,000,000		$ 900,000,000	$ 40,000,000
Borrowings Outstanding	247,000,000		139,000,000
Letters of Credit Outstanding	2,200,000	$ 93,800,000	2,300,000
Tampa Electric Company [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	475,000,000		475,000,000		$ 200,000,000
Borrowings Outstanding	61,000,000		58,000,000
Letters of Credit Outstanding	500,000		600,000
Tampa Electric Company [Member] | 5-year Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	325,000,000		325,000,000
Borrowings Outstanding	0		12,000,000
Letters of Credit Outstanding	500,000		600,000
Tampa Electric Company [Member] | 3-year Accounts Receivable Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	150,000,000		150,000,000
Borrowings Outstanding	61,000,000		46,000,000
Letters of Credit Outstanding	0		0
TECO Energy [Member] | 5-year Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	300,000,000		300,000,000
Borrowings Outstanding	163,000,000		50,000,000
Letters of Credit Outstanding	0		0
New Mexico Gas Company [Member] | 5-year Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	125,000,000		125,000,000
Borrowings Outstanding	23,000,000		31,000,000
Letters of Credit Outstanding	$ 1,700,000		$ 1,700,000