Share-Based Payment Arrangements (Detail) - Schedule of stock options outstanding and exercisable (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Number of Option Shares Outstanding	250,821	392,350	259,189	404,387
Weighted-Average Remaining Contractual Life (Years)	1 year 73 days	1 year 255 days
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 22.51	$ 23.73	$ 22.30	$ 23.80
Low Outstanding Exercise Price [Member] | Range $8.90 to $13.63 [Member] | Range Of Exercise Prices [Member]
Low Outstanding (in Dollars)	$ 8.90
Low Outstanding Exercise Price [Member] | Range $14.00 to $16.88 [Member] | Range Of Exercise Prices [Member]
Low Outstanding (in Dollars)	14.00
Low Outstanding Exercise Price [Member] | Range $17.00 to $19.54 [Member] | Range Of Exercise Prices [Member]
Low Outstanding (in Dollars)	17.00
Low Outstanding Exercise Price [Member] | Range $20.77 to $23.90 [Member] | Range Of Exercise Prices [Member]
Low Outstanding (in Dollars)	20.77
Low Outstanding Exercise Price [Member] | Range $24.26 to $28.50 [Member] | Range Of Exercise Prices [Member]
Low Outstanding (in Dollars)	24.26
Low Outstanding Exercise Price [Member] | Range $29.54 to $31.90 [Member] | Range Of Exercise Prices [Member]
Low Outstanding (in Dollars)	29.35
Low Outstanding Exercise Price [Member] | Range $32.28 to $38.71 [Member] | Range Of Exercise Prices [Member]
Low Outstanding (in Dollars)	32.28
High Outstanding Exercise Price [Member] | Range $8.90 to $13.63 [Member] | Range Of Exercise Prices [Member]
High Outstanding (in Dollars)	13.63
High Outstanding Exercise Price [Member] | Range $14.00 to $16.88 [Member] | Range Of Exercise Prices [Member]
High Outstanding (in Dollars)	16.88
High Outstanding Exercise Price [Member] | Range $17.00 to $19.54 [Member] | Range Of Exercise Prices [Member]
High Outstanding (in Dollars)	19.54
High Outstanding Exercise Price [Member] | Range $20.77 to $23.90 [Member] | Range Of Exercise Prices [Member]
High Outstanding (in Dollars)	23.90
High Outstanding Exercise Price [Member] | Range $24.26 to $28.50 [Member] | Range Of Exercise Prices [Member]
High Outstanding (in Dollars)	28.50
High Outstanding Exercise Price [Member] | Range $29.54 to $31.90 [Member] | Range Of Exercise Prices [Member]
High Outstanding (in Dollars)	31.90
High Outstanding Exercise Price [Member] | Range $32.28 to $38.71 [Member] | Range Of Exercise Prices [Member]
High Outstanding (in Dollars)	$ 38.71
Number Of Option Shares Outstanding [Member] | Range $8.90 to $13.63 [Member] | Options Outstanding [Member]
Number of Option Shares Outstanding	29,775
Number Of Option Shares Outstanding [Member] | Range $8.90 to $13.63 [Member] | Options Exercisable [Member]
Number of Option Shares Outstanding	29,775
Number Of Option Shares Outstanding [Member] | Range $14.00 to $16.88 [Member] | Options Outstanding [Member]
Number of Option Shares Outstanding	40,069
Number Of Option Shares Outstanding [Member] | Range $14.00 to $16.88 [Member] | Options Exercisable [Member]
Number of Option Shares Outstanding	22,776
Number Of Option Shares Outstanding [Member] | Range $17.00 to $19.54 [Member] | Options Outstanding [Member]
Number of Option Shares Outstanding	55,542
Number Of Option Shares Outstanding [Member] | Range $17.00 to $19.54 [Member] | Options Exercisable [Member]
Number of Option Shares Outstanding	55,542
Number Of Option Shares Outstanding [Member] | Range $20.77 to $23.90 [Member] | Options Outstanding [Member]
Number of Option Shares Outstanding	28,942
Number Of Option Shares Outstanding [Member] | Range $20.77 to $23.90 [Member] | Options Exercisable [Member]
Number of Option Shares Outstanding	28,942
Number Of Option Shares Outstanding [Member] | Range $24.26 to $28.50 [Member] | Options Outstanding [Member]
Number of Option Shares Outstanding	22,150
Number Of Option Shares Outstanding [Member] | Range $24.26 to $28.50 [Member] | Options Exercisable [Member]
Number of Option Shares Outstanding	22,150
Number Of Option Shares Outstanding [Member] | Range $29.54 to $31.90 [Member] | Options Outstanding [Member]
Number of Option Shares Outstanding	4,000
Number Of Option Shares Outstanding [Member] | Range $29.54 to $31.90 [Member] | Options Exercisable [Member]
Number of Option Shares Outstanding	4,000
Number Of Option Shares Outstanding [Member] | Range $32.28 to $38.71 [Member] | Options Outstanding [Member]
Number of Option Shares Outstanding	70,343
Number Of Option Shares Outstanding [Member] | Range $32.28 to $38.71 [Member] | Options Exercisable [Member]
Number of Option Shares Outstanding	70,343
Number Of Option Shares Outstanding [Member] | Options Outstanding [Member]
Number of Option Shares Outstanding	250,821
Number Of Option Shares Outstanding [Member] | Options Exercisable [Member]
Number of Option Shares Outstanding	233,528
Weighted Average Remaining Contractual Life [Member] | Range $8.90 to $13.63 [Member] | Options Outstanding [Member]
Weighted-Average Remaining Contractual Life (Years)	1 year 197 days
Weighted Average Remaining Contractual Life [Member] | Range $14.00 to $16.88 [Member] | Options Outstanding [Member]
Weighted-Average Remaining Contractual Life (Years)	2 years 54 days
Weighted Average Remaining Contractual Life [Member] | Range $17.00 to $19.54 [Member] | Options Outstanding [Member]
Weighted-Average Remaining Contractual Life (Years)	262 days
Weighted Average Remaining Contractual Life [Member] | Range $20.77 to $23.90 [Member] | Options Outstanding [Member]
Weighted-Average Remaining Contractual Life (Years)	1 year 54 days
Weighted Average Remaining Contractual Life [Member] | Range $24.26 to $28.50 [Member] | Options Outstanding [Member]
Weighted-Average Remaining Contractual Life (Years)	80 days
Weighted Average Remaining Contractual Life [Member] | Range $29.54 to $31.90 [Member] | Options Outstanding [Member]
Weighted-Average Remaining Contractual Life (Years)	1 year 58 days
Weighted Average Remaining Contractual Life [Member] | Range $32.28 to $38.71 [Member] | Options Outstanding [Member]
Weighted-Average Remaining Contractual Life (Years)	1 year 76 days
Weighted Average Remaining Contractual Life [Member] | Options Outstanding [Member]
Weighted-Average Remaining Contractual Life (Years)	1 year 73 days
Weighted Average Exercise Price [Member] | Range $8.90 to $13.63 [Member] | Options Outstanding [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 11.87
Weighted Average Exercise Price [Member] | Range $8.90 to $13.63 [Member] | Options Exercisable [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 11.87
Weighted Average Exercise Price [Member] | Range $14.00 to $16.88 [Member] | Options Outstanding [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 14.15
Weighted Average Exercise Price [Member] | Range $14.00 to $16.88 [Member] | Options Exercisable [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 14.15
Weighted Average Exercise Price [Member] | Range $17.00 to $19.54 [Member] | Options Outstanding [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 19.54
Weighted Average Exercise Price [Member] | Range $17.00 to $19.54 [Member] | Options Exercisable [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 19.54
Weighted Average Exercise Price [Member] | Range $20.77 to $23.90 [Member] | Options Outstanding [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 23.66
Weighted Average Exercise Price [Member] | Range $20.77 to $23.90 [Member] | Options Exercisable [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 23.66
Weighted Average Exercise Price [Member] | Range $24.26 to $28.50 [Member] | Options Outstanding [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 25.21
Weighted Average Exercise Price [Member] | Range $24.26 to $28.50 [Member] | Options Exercisable [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 25.21
Weighted Average Exercise Price [Member] | Range $29.54 to $31.90 [Member] | Options Outstanding [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 29.92
Weighted Average Exercise Price [Member] | Range $29.54 to $31.90 [Member] | Options Exercisable [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 29.92
Weighted Average Exercise Price [Member] | Range $32.28 to $38.71 [Member] | Options Outstanding [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 32.38
Weighted Average Exercise Price [Member] | Range $32.28 to $38.71 [Member] | Options Exercisable [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 32.38
Weighted Average Exercise Price [Member] | Options Outstanding [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 22.51
Weighted Average Exercise Price [Member] | Options Exercisable [Member]
Weighted-Average Exercise Price ($) (in Dollars per share)	$ 23.13