Fair Value Measurements - Schedule of Recurring Fair Value Measurements (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivatives assets	$ 2.2	$ 0.2
Natural gas derivative liabilities	1.7	26.2
Tampa Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivatives assets	1.5	0.0
Natural gas derivative liabilities	1.5	26.2
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivatives assets	0.0	0.0
Natural gas derivative liabilities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Tampa Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivatives assets	0.0	0.0
Natural gas derivative liabilities	0.0	0.0
Natural Gas Derivatives [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivatives assets	2.2	0.2
Natural gas derivative liabilities	1.7	26.2
Natural Gas Derivatives [Member] | Fair Value, Measurements, Recurring [Member] | Tampa Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivatives assets	1.5
Natural gas derivative liabilities	1.5	26.2
Natural Gas Derivatives [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivatives assets	0.0	0.0
Natural gas derivative liabilities	0.0	0.0
Natural Gas Derivatives [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Tampa Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivatives assets	0.0
Natural gas derivative liabilities	0.0	0.0
Natural Gas Derivatives [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivatives assets	2.2	0.2
Natural gas derivative liabilities	1.7	26.2
Natural Gas Derivatives [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Tampa Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivatives assets	1.5
Natural gas derivative liabilities	1.5	26.2
Natural Gas Derivatives [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivatives assets	0.0	0.0
Natural gas derivative liabilities	0.0	0.0
Natural Gas Derivatives [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Tampa Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivatives assets	0.0
Natural gas derivative liabilities	$ 0.0	$ 0.0