Fair Value of Performance Units (Detail) - Performance Units Awards $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)
2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	$ 3,854
2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	4,052
2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	5,388
2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	5,564
2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	3,065
2011
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate fair value at date of grant	$ 5,569