Summary of Option Activity Under SIPs (Detail) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2010
Options Outstanding - Number of Shares
Outstanding, Beginning	2,043,774	2,020,767
Granted	215,337
Exercised	(579,019)
Forfeited or expired	(76,758)
Outstanding, Ending	1,603,334	2,020,767
Vested or expected to vest, Ending	1,573,281
Exercisable, Ending	1,223,181
Options Outstanding - Weighted-Average Exercise Price
Outstanding, Beginning	$ 33.39
Granted	$ 35.07
Exercised	$ 33.18
Forfeited or expired	$ 37.75
Outstanding, Ending	$ 33.49
Vested or expected to vest, Ending	$ 33.55
Exercisable, Ending	$ 33.57
Options Outstanding - Weighted Average Remaining Contractual Term (Years)
Outstanding, Ending	5 years 9 months 29 days
Vested or expected to vest, Ending	5 years 9 months 15 days
Exercisable, Ending	4 years 11 months 9 days
Options Outstanding - Aggregate Intrinsic Value
Outstanding, Beginning	$ 21,430,372
Vested or expected to vest, Ending	20,932,377
Exercisable, Ending	$ 16,310,574

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