Commitments and contingencies - Additional Information (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Loss Contingencies [Line Items]
Capital Commitments	$ 1,400	$ 0
Auction Rate Securities Committed To Purchase related to settlements with regulators	3,500
Auction Rate Securities Committed To Purchase as a Result of Legal Settlements and Awards	3,700
Minimum [Member]
Loss Contingencies [Line Items]
Maximum estimated range of aggregate loss for legal proceedings	0
Maximum [Member]
Loss Contingencies [Line Items]
Maximum estimated range of aggregate loss for legal proceedings	$ 3,000
Maximum [Member] \| Auction rate securities
Loss Contingencies [Line Items]
Securities Committed To Purchase Period	2020

Auction Rate Securities Committed To Purchase as a Result of Legal Settlements and Awards

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- Definition

Auction Rate Securities Committed To Purchase related to settlements with regulators

+ References

No definition available.

+ Details

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- Definition

The amount of unfunded capital for which the entity is obligated, to those certain investments for which net asset value per share is calculated.

+ References

No definition available.

+ Details

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- Definition

Securities committed to purchase period.

+ References

No definition available.

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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.

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- Definition

The difference between the maximum amount of estimated loss and the amount recorded as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 05
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=99385795&loc=d3e12631-108344

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