Fair Value of Performance Units (Detail) - Performance Units Awards $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value at date of grant	$ 8,004
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