Employee Plans (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2013 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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	2,970,072
Purchase price of common stock	85.00%
Summary of information regarding the Common Shares issued under the ESPP
Shares issued	68,462	68,807	73,468
Employee Share Purchase Plan Issuance Price Minimum | $ / shares	$ 63.70	$ 45.90	$ 44.26
Employee Share Purchase Plan Issuance Price Maximum | $ / shares	$ 65.90	$ 55.95	$ 48.17
Issuance proceeds | $	$ 4,404,000	$ 3,392,000	$ 3,401,000
Maximum 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 | $	$ 5,500,000	$ 5,200,000	$ 4,200,000
Share-based Compensation Arrangements by Share-based Payment Award, Award Type and Plan Name [Domain]
Summary of information regarding the Common Shares issued under the ESPP
Maximum eligible compensation percentage for the defined contribution plan (the 401(k) Plan)	3.00%