Performance Units (Detail)	9 Months Ended
	Sep. 30, 2021 shares
2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Target number of shares	843,000
2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Target number of shares	500,500
2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Target number of shares	489,800
2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Target number of shares	310,700
2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Target number of shares	186,198