Employee Postretirement Benefits - Schedule of Change in Plan Assets (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Change in plan assets
Fair value of plan assets at beginning of year	$ 648.0
Employer contributions	55.0	$ 47.5
Fair value of plan assets at end of year	625.4	648.0
Pension Benefits [Member]
Change in plan assets
Fair value of plan assets at beginning of year	648.0	593.0
Actual return on plan assets	(25.5)	46.4
Employer contributions	55.0	47.5
Employer direct benefit payments	0.9	1.0
Plan participants’ contributions	0.0	0.0
Transfer in due to acquisition	0.0	0.0
Benefits paid	(53.0)	(39.9)
Fair value of plan assets at end of year	625.4	648.0
Pension Benefits [Member] \| TECO Energy [Member]
Change in plan assets
Fair value of plan assets at beginning of year	648.0	593.0
Actual return on plan assets	(25.5)	46.4
Employer contributions	55.0	47.5
Employer direct benefit payments	0.9	1.0
Plan participants’ contributions	0.0	0.0
Benefits paid	(53.0)	(39.9)
Fair value of plan assets at end of year	625.4	648.0
Transfer in due to acquisition	0.0	0.0
Other Postretirement Benefits [Member]
Change in plan assets
Fair value of plan assets at beginning of year	18.8	0.0
Actual return on plan assets	(0.6)	0.1
Employer contributions	1.5	(1.0)
Employer direct benefit payments	13.5	16.0
Plan participants’ contributions	2.0	2.8
Transfer in due to acquisition	0.0	16.9
Benefits paid	(14.6)	(16.0)
Fair value of plan assets at end of year	20.6	18.8
Other Postretirement Benefits [Member] \| TECO Energy [Member]
Change in plan assets
Fair value of plan assets at beginning of year	18.8	0.0
Actual return on plan assets	(0.6)	0.1
Employer contributions	1.5	(1.0)
Employer direct benefit payments	13.5	16.0
Plan participants’ contributions	2.0	2.8
Benefits paid	(14.6)	(16.0)
Fair value of plan assets at end of year	20.6	18.8
Transfer in due to acquisition	$ 0.0	$ 16.9

Defined Benefit Plan Direct Benefit Payments By Employer

No definition available.

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- Definition

The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Actual Return on Plan Assets (Component of Net Periodic Postretirement Benefit Cost)
-URI http://asc.fasb.org/extlink&oid=6504192

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Actual Return on Plan Assets (Component of Net Periodic Pension Cost)
-URI http://asc.fasb.org/extlink&oid=6504226

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- Definition

Fair value of assets transferred into (out of) a defined benefit pension plan.

+ References

No definition available.

+ Details

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- Definition

The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 60
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Benefits
-URI http://asc.fasb.org/extlink&oid=49170737

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(6)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Postretirement Benefits
-URI http://asc.fasb.org/extlink&oid=6521376

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(5)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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- Definition

The amount of increase in the plan assets attributed to a business combination.

+ References

+ Details

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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- Definition

The increase in the fair value of plan assets from contributions made by the employer.

+ References

+ Details

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- Definition

The amount of contributions made by plan participants. This item represents a periodic increase to the plan obligation and an increase to plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(4)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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- Definition

Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 30
-Section 35
-Paragraph 50
-URI http://asc.fasb.org/extlink&oid=49178875&loc=d3e12355-114930

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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