Basis of Presentation - Schedule of Vesting Information Stock Options Outstanding (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Stock Options:
Unvested at beginning of period, Shares	157,298	112,258
Granted, Shares	122,357	32,830
Vested, Shares	(41,794)	(34,951)
Unvested at end of period, Shares	237,861	110,137
Unvested at beginning of period, Weighted-Average Exercise Price	$ 38.57	$ 36.08
Granted, Weighted-Average Exercise Price	$ 47.50	$ 40.89
Vested, Weighted-Average Exercise Price	$ 34.30	$ 32.89
Unvested at end of period, Weighted-Average Exercise Price	$ 43.92	$ 38.53