Short-Term Debt - Additional Information (Detail) - USD ($)				12 Months Ended
	Nov. 02, 2017	Mar. 22, 2017	Mar. 24, 2015	Dec. 31, 2017	Dec. 31, 2016
Line Of Credit Facility [Line Items]
Weighted-average interest rate				2.07%	1.49%
Line of credit facility maximum borrowing capacity		$ 50,000,000		$ 775,000,000	$ 475,000,000
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
364-day Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Credit facility maturity period	364 days
Line of credit facility maximum borrowing capacity	$ 300,000,000
Credit facility maturity date	Nov. 01, 2018
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
Credit facility maturity date			Mar. 23, 2018	Mar. 23, 2018	Mar. 23, 2018
Credit facility amendment date			Mar. 24, 2015
Loan agreement program and liquidity fees				0.65%
Interest rate description				TEC will pay program and liquidity fees, which total 65 basis points as of December 31, 2017. 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 either the BTMU’s prime rate, the federal funds rate, or the London interbank deposit rate, plus a margin
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		Dec. 17, 2018
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		Mar. 22, 2022