Share based compensation plans - Schedule of Stock Options Outstanding and Exercisable by Price Range (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012
$10.42 - $25.01 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	$ 10.42
Exercise price, high end of range (in dollars per share)	$ 25.01
Number outstanding (in shares)	253,661
Weighted Average Remaining Contractual Life of options outstanding (in years)	5 years 1 month 2 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 23.40
Number Exercisable (in shares)	253,661
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 23.40
$25.05 - $28.95 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	$ 25.05
Exercise price, high end of range (in dollars per share)	$ 28.95
Number outstanding (in shares)	326,383
Weighted Average Remaining Contractual Life of options outstanding (in years)	5 years 3 months 18 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 26.96
Number Exercisable (in shares)	314,383
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 26.97
$29.23 - $33.01 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	$ 29.23
Exercise price, high end of range (in dollars per share)	$ 33.01
Number outstanding (in shares)	314,433
Weighted Average Remaining Contractual Life of options outstanding (in years)	6 years 10 months 2 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 31.60
Number Exercisable (in shares)	167,102
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 31.44
$33.08 - $38.11 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	$ 33.08
Exercise price, high end of range (in dollars per share)	$ 38.11
Number outstanding (in shares)	347,859
Weighted Average Remaining Contractual Life of options outstanding (in years)	3 years 6 months
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 37.49
Number Exercisable (in shares)	307,194
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 37.51
$38.40 - $39.94 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	$ 38.40
Exercise price, high end of range (in dollars per share)	$ 39.94
Number outstanding (in shares)	282,687
Weighted Average Remaining Contractual Life of options outstanding (in years)	6 years 7 months 10 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 39.68
Number Exercisable (in shares)	126,687
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 39.69
$40.27 - $41.37 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	$ 40.27
Exercise price, high end of range (in dollars per share)	$ 41.37
Number outstanding (in shares)	210,000
Weighted Average Remaining Contractual Life of options outstanding (in years)	7 years 18 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 41.22
Number Exercisable (in shares)	68,334
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 41.20
$38.11 - $39.24 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	$ 43.04
Exercise price, high end of range (in dollars per share)	$ 45.84
Number outstanding (in shares)	188,932
Weighted Average Remaining Contractual Life of options outstanding (in years)	3 years 5 months 19 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 44.63
Number Exercisable (in shares)	188,932
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 44.63
$43.04 - $45.84 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	$ 50.15
Exercise price, high end of range (in dollars per share)	$ 50.15
Number outstanding (in shares)	12,500
Weighted Average Remaining Contractual Life of options outstanding (in years)	4 years 3 months 15 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 50.15
Number Exercisable (in shares)	12,500
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 50.15
$50.15 - $50.15 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	$ 50.50
Exercise price, high end of range (in dollars per share)	$ 50.50
Number outstanding (in shares)	2,000
Weighted Average Remaining Contractual Life of options outstanding (in years)	4 years 4 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 50.50
Number Exercisable (in shares)	2,000
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 50.50
$50.50 - $50.50 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	$ 50.99
Exercise price, high end of range (in dollars per share)	$ 50.99
Number outstanding (in shares)	7,500
Weighted Average Remaining Contractual Life of options outstanding (in years)	4 years 15 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 50.99
Number Exercisable (in shares)	7,500
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 50.99
$50.99 - $50.99 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	$ 10.42
Exercise price, high end of range (in dollars per share)	$ 50.99
Number outstanding (in shares)	1,945,955
Weighted Average Remaining Contractual Life of options outstanding (in years)	5 years 4 months 21 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 34.50
Number Exercisable (in shares)	1,448,293
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 33.54