Share Based Compensation (Summary of Share Option Activity) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Number of Common Shares
Balance at January 1, 2014	307,800
Granted	21,251
Exercised	(13,834)	(10,964)	(3,906)
Forfeited or expired	(90,836)
Balance at December 31, 2014	224,381	307,800
Exercisable, Common Shares	183,227
Weighted Average Exercise Share Price
Balance at January 1, 2014	$ 25.23
Granted	$ 24.75
Exercised	$ 13.72
Forfeited or expired	$ 37.62
Balance at December 31, 2014	$ 20.88	$ 25.23
Exercisable at December 31, 2014	$ 20.54
Weighted average fair value of options granted during the year	$ 8.05	$ 7.99	$ 7.85
Weighted Average Contractual Remaining Life(Years)
Balance at end of period	2 years 4 months 24 days
Exercisable at end of period	1 year 8 months 12 days
Aggregate Intrinsic Value
Balance at January 1, 2014	$ 1,768,000
Granted	0
Exercised	147,000
Forfeited or expired	0
Balance at December 31, 2014	1,300,000	1,768,000
Exercisable, Intrinsic Value	$ 1,148,000