Fair Value of Performance Units (Detail) - Performance Units Awards $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($)
2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value at date of grant	$ 3,854
2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value at date of grant	4,052
2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value at date of grant	5,388
2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value at date of grant	5,564
2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value at date of grant	$ 3,065