Fair Value of Performance Units (Detail) - Performance Units Awards $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($)
2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value at date of grant	$ 5,780
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