Asset Retirement Obligations - Schedule of Asset Retirement Obligations (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Asset Retirement Obligation [Line Items]
Beginning balance	$ 6.1	$ 28.6
Additional liabilities	0.9	0.1
Revisions to estimated cash flows	(0.5)	0.2
Acquisition of NMGC	0.0	0.8
Reclassification to liabilities associated with assets held for sale	0.0	(22.5)
Other	0.3	(1.1)
Ending balance	6.8	6.1
Tampa Electric Company [Member]
Asset Retirement Obligation [Line Items]
Beginning balance	5.3	4.8
Additional liabilities	0.9	0.1
Revisions to estimated cash flows	(0.5)	0.2
Other	0.3	0.2
Ending balance	$ 6.0	$ 5.3