Commitments and Contingencies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 29, 2017 Worker
Long Term Commitments [Line Items]
Total rental expense and lease | $	$ 2	$ 2	$ 4
PGS [Member]
Long Term Commitments [Line Items]
Ultimate financial liability to superfund sites and former MGP sites | $	$ 30
Industrial Accident [Member]
Long Term Commitments [Line Items]
Number of workers killed				5
Number of workers sustained serious injuries				1
Industrial Accident [Member] | Tampa Electric Employee [Member]
Long Term Commitments [Line Items]
Number of workers killed				1
Industrial Accident [Member] | Contract Workers [Member]
Long Term Commitments [Line Items]
Number of workers killed				4