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Employee Plans (Details) | 12 Months Ended
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Dec. 31, 2016 USD ($) $ / shares shares | Dec. 31, 2015 USD ($) $ / shares shares | Dec. 31, 2014 USD ($) $ / shares shares | Dec. 31, 2013
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 | shares | 7,000,000 |  |  |
Common Shares available for purchase under the ESPP | shares | 2,906,163 |  |  |
Purchase price of common stock | 85.00% |  |  |
Summary of information regarding the Common Shares issued under the ESPP |  |  |  |
Shares issued | shares | 63,909 | 68,462 | 68,807 |
Employee Share Purchase Plan Issuance Price Minimum | $ / shares | $ 51.85 | $ 63.70 | $ 45.90 |
Employee Share Purchase Plan Issuance Price Maximum | $ / shares | $ 63.37 | $ 65.90 | $ 55.95 |
Issuance proceeds | $ | $ 3,686,000 | $ 4,404,000 | $ 3,392,000 |
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 | $ | $ 4,900,000 | $ 5,500,000 | $ 5,200,000 |
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% |  |  |

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- Definition Description of the period of time over which an employee's right to contribution plus actual earnings is no longer contingent on satisfaction of a service condition. + References No definition available. + Details
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- Definition Represents the maximum price that employees purchase common stock for under the employee purchase plan during the year. + References No definition available. + Details
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- Definition Represents the minimum price that employees purchase common stock for under the employee purchase plan during the year. + References No definition available. + Details
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- Definition Represents the maximum amount of an employee's contribution to the defined contribution plan that is matched by the company. + References No definition available. + Details
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- Definition Share Based Compensation Arrangement By Share Based Payment Award information regarding the Common Shares issued under the Employee Stock Purchase Plan. + References No definition available. + Details
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- Definition Represents the number of common shares available for purchase under the employee share purchase plan at the balance sheet date. + References No definition available. + Details
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- Definition Represents the maximum value of common shares an employee can acquire under the employee share purchase plan on an annual basis. + References No definition available. + Details
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- Definition Represents the number of shares authorized for purchase under the employee share purchase plan after the most recent amendment. + References No definition available. + Details
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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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- Definition The cash inflow associated with the amount received from holders to acquire the entity's shares under incentive and share awards other than stock option exercises. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately. + References No definition available. + Details
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- Definition Discount rate from fair value on purchase date that participants pay for shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Number of share instruments newly issued under a share-based compensation plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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