Employee Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of information regarding the Common Shares issued under the ESPP
Shares issued	68,807	73,468	110,054
Employee Share Purchase Plan Issuance Price Minimum	$ 45.90	$ 44.26	$ 46.33
Employee Share Purchase Plan Issuance Price Maximum	$ 55.95	$ 48.17	$ 51.78
Issuance proceeds	$ 3,392,000	$ 3,401,000	$ 5,399,000
Value of common shares that can be acquired by the employee and trustees under employees share purchase plan	100,000
Number of common shares authorized after amendment	7,000,000
Number of common shares authorized before amendment	2,000,000
Common Shares available for purchase under the ESPP	3,038,534
Purchase price of common stock	85.00%
Minimum eligible compensation percentage for the defined contribution plan (the "401(k) Plan")	4.00%	3.00%
Defined contribution plan vesting period	5
Defined contribution plan, compensation expense	$ 5,200,000	$ 4,200,000	$ 4,400,000