Fair Value Measurements - Schedule of Recurring Fair Value Measurements (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivative assets	$ 0.2	$ 0.0
Natural gas derivative liabilities	26.6	42.7
Tampa Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivative assets	0.0	0.0
Natural gas derivative liabilities	26.6	42.7
Fair Value, Measurements, Recurring [Member] | Natural Gas Derivatives [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivative assets	0.2
Natural gas derivative liabilities	26.6	42.7
Fair Value, Measurements, Recurring [Member] | Natural Gas Derivatives [Member] | Tampa Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivative liabilities	26.6	42.7
Fair Value, Measurements, Recurring [Member] | Natural Gas Derivatives [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivative assets	0.0
Natural gas derivative liabilities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Natural Gas Derivatives [Member] | Level 1 [Member] | Tampa Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivative liabilities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Natural Gas Derivatives [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivative assets	0.2
Natural gas derivative liabilities	26.6	42.7
Fair Value, Measurements, Recurring [Member] | Natural Gas Derivatives [Member] | Level 2 [Member] | Tampa Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivative liabilities	26.6	42.7
Fair Value, Measurements, Recurring [Member] | Natural Gas Derivatives [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivative assets	0.0
Natural gas derivative liabilities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Natural Gas Derivatives [Member] | Level 3 [Member] | Tampa Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas derivative liabilities	$ 0.0	$ 0.0