Share-Based Compensation - Schedule of Share and Dividend Equivalent Share Award Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Total Share Equivalents [Roll Forward]
Beginning balance (in shares)	462,430	475,347
Granted (in shares)	192,359	147,460
Forfeited share equivalents (in shares)	(16,283)	(24,310)
Vested share equivalents (in shares)	(147,511)	(127,874)
Ending balance (in shares)	490,995	470,623
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Value of Share Equivalents [Roll Forward]
Beginning balance	$ 25,466	$ 30,826
Granted	11,490	10,061
Forfeited share equivalents	973	1,659
Vested share equivalents	7,452	8,087
Ending balance	$ 29,327	$ 32,111
Market price of Company's stock (in usd per share)	$ 59.73	$ 68.23