Share-Based Compensation - Schedule of Share and Dividend Equivalent Share Award Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Share Equivalents [Roll Forward]
Beginning balance (in shares)	460,212	417,238
Granted (in shares)	156,505	115,620
Forfeited share equivalents (in shares)	(29,180)	(19,498)
Vested share equivalents (in shares)	(130,380)	(68,480)
Ending balance (in shares)	457,157	444,880
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Dividend Equivalents [Roll Forward]
Beginning balance (in shares)	22,648	22,351
Granted (in shares)	7,261	7,147
Forfeited share (in shares)	(1,541)	(1,751)
Vested share equivalents (in shares)	(9,134)	(6,615)
Ending balance (in shares)	19,234	21,132
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Total Share Equivalents [Roll Forward]
Beginning balance (in shares)	482,860	439,589
Granted (in shares)	163,766	122,767
Forfeited share equivalents (in shares)	(30,721)	(21,249)
Vested share equivalents (in shares)	(139,514)	(75,095)
Ending balance (in shares)	476,391	466,012
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Value of Share Equivalents [Roll Forward]
Beginning balance (in shares)	$ 31,313	$ 27,628
Granted (in shares)	9,533	7,674
Forfeited share equivalents (in shares)	(1,788)	(1,328)
Vested share equivalents (in shares)	(8,829)	(5,002)
Ending balance (in shares)	$ 27,731	$ 29,130
Market price of Company's stock (in usd per share)	$ 58.21	$ 62.51