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Commitments and contingencies Textual (Details) - USD ($) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Other Commitments [Line Items] |  |  |  |  |
Operating Leases, Rent Expense | $ 2,300,000 | $ 2,100,000 | $ 7,100,000 | $ 6,300,000 |
Operating Leases, Future Minimum Payments Due | 0 |  | 0 |  |
Cash Reserve Deposit Required and Made | $ 34,900,000 |  | $ 34,900,000 |  | $ 35,800,000
Minimum [Member] |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Operating Leases, Lease Renewal Period Option |  |  | 5 years |  |
Maximum [Member] |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Operating Leases, Lease Renewal Period Option |  |  | 10 years |  |

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- Definition Cash deposited in a special reserve account for the exclusive benefit of customers pursuant to SEC Regulations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 305 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6478312&loc=d3e61473-112792 + Details
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- Definition Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line. + References No definition available. + Details
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- Definition Amount of required minimum rental payments for leases having an initial or remaining non-cancelable letter-terms in excess of one year. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=82846649&loc=d3e38371-112697 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41502-112717 + Details
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