Summary of Market- Based Awards Granted (Detail) - shares	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares Granted	687,685	585,833
Market Based Share Awards | 2016 Market Based Share Plan | Performance Period Ending December 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares Granted	6,403
Period Expensed Through	2017-12
Performance Measure	TSR
Market Based Share Awards | 2016 Market Based Share Plan | Performance Period Ending December 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares Granted	50,878
Period Expensed Through	2018-12
Performance Measure	TSR
Market Based Share Awards | 2015 Market-Based Share Plan | Performance Period Ending December 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares Granted		54,872
Period Expensed Through		2017-12
Performance Measure	TSR