Employee Postretirement Benefits - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Primary provisions for pension benefits	First, for plans funded less than 100%, required shortfall contributions will be based on a percentage of the funding target until 2012, rather than the funding target of 100%.
Percentage of qualified pension plan's actuarial value of assets		83.70%
Estimated percentage of qualified pension plan's actuarial value of assets	94.40%
Percentage of tax-free subsidy under prescription drug programs	28.00%
Tax of premiums paid for retiree medical benefits plan	40.00%
Accumulated benefit obligation of defined benefit pension plans	$ 664.7	$ 596.2
Percentage of defined benefit plan actual return on plan assets	12.60%
Employer contributions	35.5
Employer contributions in next fiscal year	15.1
Defined contribution plan, maximum employer match percentage	6.00%
Description of defined contribution plan	Employer matching contributions were 60% of eligible participant contributions with additional incentive match of up to 40% of eligible participant contributions based on the achievement of certain operating company financial goals. Prior to this, the employer matching contributions were 50% of eligible participant contributions, with an additional incentive match of up to 50%
Defined contribution plan cost recognized	7.0	9.0	12.6
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Range of estimated annual contributions	30.0
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Range of estimated annual contributions	50.0
TAMPA ELECTRIC CO [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan cost recognized	6.0	5.8	8.8
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	1.3	3.7
Employer contributions in next fiscal year	5.3
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Other postretirement, prior service cost amortized from accumulated other comprehensive income(loss) in next fiscal year	0.2
Other postretirement, transition obligation amortized from accumulated other comprehensive income (loss) in next fiscal year	0.3
Other postretirement benefit plans, Net loss that will be amortized from regulatory assets in next fiscal year	0.9
Employer contributions	9.8	11.2
Employer contributions in next fiscal year	13.1
Net periodic benefit cost	15.2	16.4	17.2
Other Postretirement Benefits [Member] | TAMPA ELECTRIC CO [Member]
Defined Benefit Plan Disclosure [Line Items]
Other postretirement benefit plans, Net loss that will be amortized from regulatory assets in next fiscal year	0.9
Employer contributions in next fiscal year	10.6
Net periodic benefit cost	12.4	10.0	13.8
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated net loss amortized from regulatory assets into net periodic benefit cost	4.4
Prior service net cost amortized from regulatory assets into net periodic benefit cost	0.1
Defined benefit plan net loss that will be amortized from regulatory assets in next fiscal year	15.7
Defined benefit plan prior service benefit that will be amortized from regulatory assets in next fiscal year	0.5
Employer contributions	36.8	3.7
Net periodic benefit cost	24.9	20.3	26.7
Pension Benefits [Member] | TAMPA ELECTRIC CO [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan net loss that will be amortized from regulatory assets in next fiscal year	15.7
Defined benefit plan prior service benefit that will be amortized from regulatory assets in next fiscal year	0.5
Net periodic benefit cost	$ 18.3	$ 13.1	$ 18.6