Compensation Expense Associated with Performance Units (Detail) - Performance Units Awards - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense associated with Performance Units	$ 321		$ 321
2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense associated with Performance Units	321	$ 321	642	$ 321
2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense associated with Performance Units	$ 338	338	675	675
2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense associated with Performance Units		$ 449	$ 449	898
2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense associated with Performance Units				$ 464