Related Party Transactions - Schedule of Related Parties (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Natural gas sales to/(from) affiliates	$ (4)	$ 0	$ 1
Services received from affiliates	67	66	69
Dividends to TECO Energy	292	289
Equity contributions from TECO Energy	190	150
Accounts receivable	2	7
Accounts payable	19	18
Taxes receivable	3	0
Taxes payable	2	7
TECO Energy [Member]
Related Party Transaction [Line Items]
Dividends to TECO Energy	292	289	268
Equity contributions from TECO Energy	$ 190	$ 150	$ 175

Natural gas sales to (from) affiliates.

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Carrying value as of the balance sheet date of liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business.

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For an unclassified balance sheet, the amount due from customers or clients for goods or services that have been delivered or sold in the normal course of business, reduced to their estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

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Carrying amount due within one year of the balance sheet date (or one operating cycle, if longer) from tax authorities as of the balance sheet date representing refunds of overpayments or recoveries based on agreed-upon resolutions of disputes.

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The cash inflow from parent as a source of financing that is recorded as additional paid in capital.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Carrying value as of the balance sheet date of obligations incurred and payable for statutory income, sales, use, payroll, excise, real, property and other taxes.

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