Employee Postretirement Benefits - Schedule of Fair Value Hierarchy Plan's Investments (Parenthetical) (Detail) - TECO Energy [Member] - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 649.4	$ 625.4
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	84.8	104.1
Common and Collective Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	270.2	171.6
Common and Collective Trusts [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0.0	0.0
Previously Reported [Member] | Common and Collective Trusts [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 53.7