Fair Value of Performance Units (Detail) - Performance Units Awards $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)
2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	$ 826
2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	9,958
2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	8,004
2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	5,780
2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	3,854
2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	$ 4,052