Share based compensation plans - Schedule of Changes in Restricted Stock (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012
Nonvested Share Activity [Line Items]
Non-vested at the beginning of the period (in shares)	69,000
Granted (in shares)	149,500
Vested (in shares)	(28,336)
Cancelled (in shares)	(29,333)
Non-vested at the end of the period (in shares)	160,831
Non-vested at the beginning of period (in dollars per share)	$ 29.23
Granted (in dollars per share)	$ 40.93
Vested (in dollars per share)	$ 29.23
Cancelled (in dollars per share)	$ 38.87
Non-vested at the end of period (in dollars per share)	$ 38.34