Fair Value of Performance Units (Detail) - Performance Units Awards	9 Months Ended
	Sep. 30, 2021 USD ($)
2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	$ 7,225,000
2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	826,000
2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	9,958,000
2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	8,004,000
2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	$ 5,780,000