Cash Flow Information - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2018
Other Significant Noncash Transactions [Line Items]
Cash paid for interest		$ 41.9	$ 43.2
Net of capitalized interest		7.0	4.7
Line of credit facilities gross borrowings		181.0	37.0
Line of credit facilities gross repayments		33.0	53.0
Increase (decrease) in accrued construction costs		$ (15.7)	$ 1.5
Common Stock
Other Significant Noncash Transactions [Line Items]
Conversion of units, shares issued (in shares)	6,250,000