Short-Term Debt - Additional Information (Detail) - USD ($)		6 Months Ended	12 Months Ended
	Mar. 23, 2018	Jun. 30, 2018	Dec. 31, 2017
Line Of Credit Facility [Line Items]
Weighted-average interest rate		2.70%	2.10%
Line of credit facility maximum borrowing capacity		$ 775,000,000	$ 775,000,000
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
Credit facility amendment date	Mar. 23, 2018
Credit facility maturity date	Mar. 22, 2021	Mar. 22, 2021	Mar. 22, 2021
Loan agreement program and liquidity fees		0.70%
Interest rate description		TEC will pay program and liquidity fees, which total 70 basis points at June 30, 2018. 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 Bank of Tokyo-Mitsubishi UFJ, Ltd.’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%
Maximum [Member]
Line Of Credit Facility [Line Items]
Commitment fees, percentage		0.35%