Share-Based Compensation - Schedule of Share and Dividend Equivalent Share Award Activity (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of share equivalents, Beginning Balance	318,729	232,921
Number of share equivalents, Granted	158,213	97,927
Number of share equivalents, Forfeited share equivalents	(5,176)	(2,191)
Number of share equivalents, Vested share equivalents	(30,697)	(5,483)
Number of share equivalents, Ending Balance	441,069	323,174
Dividend equivalents, Beginning Balance	16,035	8,942
Dividend equivalents, Granted	3,092	2,109
Dividend equivalents, Forfeited share equivalents	(243)	(95)
Dividend equivalents, Vested share equivalents	(2,120)	(350)
Dividend equivalents, Ending Balance	16,764	10,606
Total share equivalents, Beginning Balance	334,764	241,863
Total share equivalents, Granted	161,305	100,036
Total share equivalents, Forfeited share equivalents	(5,419)	(2,286)
Total share equivalents, Vested share equivalents	(32,817)	(5,833)
Total share equivalents, Ending Balance	457,833	333,780
Value of share equivalents, Beginning Balance	$ 16,444,000	$ 11,924,000
Value of share equivalents, Granted	8,068,000	5,349,000
Value of share equivalents, Forfeited share equivalents	(271,000)	(122,000)
Value of share equivalents, Vested share equivalents	(1,655,000)	(309,000)
Value of share equivalents, Ending Balance	$ 22,901,000	$ 17,847,000