Share Based Compensation (Summary of Share Option Activity) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Number of Common Shares
Balance at January 1, 2015	224,381
Granted	21,245
Exercised	(5,752)	(6,671)
Forfeited or expired	(71,721)
Balance at March 31, 2015	168,153
Exercisable, Common Shares	113,747
Weighted Average Exercise Share Price
Balance at January 1, 2015	$ 20.88
Granted	$ 26.2
Exercised	$ 12.81
Forfeited or expired	$ 27.86
Balance at March 31, 2015	$ 18.85
Exercisable at December 31, 2014	$ 16.26
Weighted average fair value of options granted during the year	$ 7.63	$ 8.05
Weighted Average Contractual Remaining Life(Years)
Balance at end of period	3 years 8 months 12 days
Exercisable at end of period	2 years 6 months
Balance at January 1, 2015	$ 1,300,000
Exercised	76,000
Balance at March 31, 2015	1,241,000
Exercisable, Intrinsic Value	$ 1,134,000