Performance Units (Detail)	12 Months Ended
	Dec. 31, 2016 shares
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
2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Target number of shares	192,000
2011
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Target number of shares	144,375