Employee Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended			21 Months Ended	24 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Percentage of tax-free subsidy under prescription drug programs		28.00%
Amortization period of Regulatory Asset		12 years
Tax of premiums paid for retiree medical benefits plan		40.00%
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 2013, rather than the funding target of 100%.
Percentage of qualified pension plan's actuarial value of assets	118.00%	118.00%	119.50%			118.00%
Employer contributions		$ 30.9	$ 43.9
Employer contributions in next fiscal year		$ 36.3
Description of defined contribution plan		Effective January 1, 2015, the employer matching contributions were 70% of eligible participant contributions with additional incentive match of up to 30% of eligible participant contributions based on the achievement of certain operating company financial goals. During the period from April 2013 to December 2014, employer matching contributions were 67% of eligible participant contributions with additional incentive match of up to 35% of eligible participant contributions based on the achievement of certain operating company financial goals. Prior to this, the employer matching contributions were 60% of eligible participant contributions with an additional incentive match of up to 40% of eligible participant contributions based on the achievement of certain operating company financial goals
Defined contribution plan cost recognized		$ 8.3	7.5	$ 10.2
Employer matching contribution percentage of eligible participant contribution					67.00%	70.00%
Defined benefit plan additional percentage of eligible compensation for matching contributions by employer					35.00%	30.00%
Pre Amendment [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer matching contribution percentage of eligible participant contribution		60.00%
Defined benefit plan additional percentage of eligible compensation for matching contributions by employer		40.00%
Unqualified SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	$ 40.8	$ 40.8	43.5			$ 40.8
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		13.3	13.5	14.8
Defined benefit plan net loss that will be amortized from regulatory assets in next fiscal year		12.7
Increase in net periodic benefit cost	1.0
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		6.4	5.7	10.4
Other postretirement benefit plans service benefit that will be amortized from regulatory assets in next fiscal year		1.8
Increase in net periodic benefit cost	0.4
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		0.0	14.9
TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation of defined benefit pension plans	723.9	$ 723.9	686.9			723.9
Percentage of defined benefit plan's assets increased		9.20%
Defined contribution plan, maximum employer match percentage		6.00%
TECO Energy [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Range of estimated annual contributions	0.5	$ 0.5				0.5
TECO Energy [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Range of estimated annual contributions	$ 29.5	29.5				$ 29.5
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		22.4	22.8	25.7
Curtailment loss		1.3	0.0	3.9
Employer contributions		37.4	55.0
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		7.0	6.5	12.6
Curtailment loss		0.0	0.0	$ (0.2)
Employer contributions		(2.6)	$ (2.1)
Employer contributions in next fiscal year		9.5
TECO Energy [Member] | Other Postretirement Benefits Florida-Based Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other postretirement benefit plans service benefit that will be amortized from regulatory assets in next fiscal year		$ 0.0