Short-Term Debt - Additional Information (Detail) - USD ($)				12 Months Ended
	Dec. 31, 2016	Mar. 24, 2015	Dec. 17, 2013	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2014
Line Of Credit Facility [Line Items]
Weighted-average interest rate	1.49%			1.49%	0.89%
Line of credit facility maximum borrowing capacity	$ 475,000,000		$ 200,000,000	$ 475,000,000	$ 475,000,000
Interest rate description				TEC to borrow funds at an interest rate equal to a margin plus the higher of Citibank's prime rate, the federal funds rate plus 50 basis points, or the London interbank deposit rate plus 1.00%;
Increase of credit facility			175,000,000
Amended And Restated Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity			$ 325,000,000			$ 325,000,000
3-year Accounts Receivable Facility [Member]
Line Of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity	$ 150,000,000	$ 150,000,000		$ 150,000,000	$ 150,000,000
Credit facility amendment date		Mar. 24, 2015
Credit facility maturity date		Mar. 23, 2018		Mar. 23, 2018	Mar. 23, 2018
Loan agreement program and liquidity fees	0.65%
Basis spread on federal funds rate		0.50%
Interest rate description				Pursuant to the Loan Agreement, TEC will pay program and liquidity fees, which total 65 basis points as of December 31, 2016. Interest rates on the borrowings are based on prevailing asset-backed commercial paper rates, unless such rates are not available from conduit lenders, in which case the rates will be at an interest rate equal to, at TEC’s option, either the BTMU’s prime rate (or the federal funds rate plus 50 basis points, if higher) or a rate based on the London interbank deposit rate (if available) plus a margin
Before Amendment and Restatement | 3-year Accounts Receivable Facility [Member]
Line Of Credit Facility [Line Items]
Credit facility maturity date		Apr. 14, 2015
Minimum [Member]
Line Of Credit Facility [Line Items]
Commitment fees, percentage				0.125%
Minimum [Member] | Amended And Restated Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Debt instrument maturity date			Oct. 25, 2016
Maximum [Member]
Line Of Credit Facility [Line Items]
Commitment fees, percentage				0.30%
Maximum [Member] | Amended And Restated Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Debt instrument maturity date			Dec. 17, 2018