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Defined contribution plans (Details 11) - USD ($) | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Nonqualified Defined Contribution Plan [Member] |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized | $ 395,000 | $ 207,000 | $ 104,000
Defined Contribution Plan [Member] |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized | $ 40,900,000 | $ 36,800,000 | $ 34,400,000

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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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Balance Type: | debit
Period Type: | duration

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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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Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |