Performance Units (Detail)	3 Months Ended
	Mar. 31, 2019 shares
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
2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Target number of shares	185,000
2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Target number of shares	190,600