Segment Information - Schedule of Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 619	$ 644	$ 1,216	$ 1,197
Total interest charges	29	30	59	59
Net income	85	86	148	142
Total assets	8,664		8,664		$ 8,364
Revenues - External [Member]
Segment Reporting Information [Line Items]
Total revenues	619	644	1,216	1,197
Intracompany Sales [Member]
Segment Reporting Information [Line Items]
Intracompany sales	0	0	0	0
Eliminations [Member]
Segment Reporting Information [Line Items]
Total revenues	(6)	(2)	(12)	(4)
Total interest charges	0	0	0	0
Net income	0	0	0	0
Total assets	(508)		(508)		(555)
Eliminations [Member] | Revenues - External [Member]
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Eliminations [Member] | Intracompany Sales [Member]
Segment Reporting Information [Line Items]
Intracompany sales	(6)	(2)	(12)	(4)
Tampa Electric [Member]
Segment Reporting Information [Line Items]
Total revenues	510		971
Tampa Electric [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	510	542	971	985
Total interest charges	25	26	51	52
Net income	74	76	121	119
Total assets	7,853		7,853		7,635
Tampa Electric [Member] | Operating Segments [Member] | Revenues - External [Member]
Segment Reporting Information [Line Items]
Total revenues	509	542	970	984
Tampa Electric [Member] | Operating Segments [Member] | Intracompany Sales [Member]
Segment Reporting Information [Line Items]
Intracompany sales	1	0	1	1
PGS [Member]
Segment Reporting Information [Line Items]
Total revenues	115		257
PGS [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	115	104	257	216
Total interest charges	4	4	8	7
Net income	11	10	27	23
Total assets	1,319		1,319		$ 1,284
PGS [Member] | Operating Segments [Member] | Revenues - External [Member]
Segment Reporting Information [Line Items]
Total revenues	110	102	246	213
PGS [Member] | Operating Segments [Member] | Intracompany Sales [Member]
Segment Reporting Information [Line Items]
Intracompany sales	$ 5	$ 2	$ 11	$ 3