Performance Units (Detail)	9 Months Ended
	Sep. 30, 2017 shares
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
2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Target number of shares	154,000
2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Target number of shares	236,500