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Savings Plan - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Savings Plan [Line Items] |  |  |
Savings plan maximum percentage of employees contribution allowable | 20.00% |  |
Percentage of employer contributions vested per year | 25.00% |  |
Maximum employment period considered for vesting of employers' contribution, in years | 4 years |  |
U.S. Qualified Pension Plan |  |  |
Savings Plan [Line Items] |  |  |
Maximum percentage of matching contributions made by the employer | 100.00% | 100.00% | 100.00%
Percentage of employer match on employee contribution | 4.00% | 4.00% | 4.00%
Savings Plan |  |  |
Savings Plan [Line Items] |  |  |
Amounts charged to statements of operations | $ 15.3 | $ 14.5 | $ 13.5

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- Definition Savings Plan [Line Items] + References No definition available. + Details
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- Definition Savings plan maximum percentage of employees contribution allowable + References No definition available. + Details
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- Definition Amount of cost for defined contribution plan. + 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 Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
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- Details
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