Significant Accounting Policies - Schedule of Fuel Inventory (Detail) - Fuel [Member] - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Inventory [Line Items]
Fuel inventory	$ 113.4	$ 96.4
Tampa Electric Company [Member]
Inventory [Line Items]
Fuel inventory	105.6	85.2
NMGC [Member]
Inventory [Line Items]
Fuel inventory	$ 7.8	$ 11.2

The period end amount for a type of inventory held by the utility in a schedule of inventories.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Inventory
-URI http://asc.fasb.org/extlink&oid=6516093

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-05.6(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_PublicUtilitiesInventory
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant