Commitments and contingencies - Capital leases (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Analysis of the leased property under capital leases
Total capital leases, gross	$ 2,465,351	$ 2,279,931
Less: Accumulated depreciation	(1,415,472)	(1,039,927)
Total capital leases, net	1,049,879	1,240,004
Future minimum rental payments under capital leases
2016	311,472
2017	160,563
2018	43,210
Total minimum lease payments	515,245
Net minimum lease payments	515,245
Less: Amounts representing interest payments	(28,134)
Present value of net minimum lease payments	487,111
Computer hardware
Analysis of the leased property under capital leases
Total capital leases, gross	2,145,668	1,989,836
Computer software
Analysis of the leased property under capital leases
Total capital leases, gross	$ 319,683	$ 290,095

Amount before accumulated depreciation of leased physical assets used in the normal conduct of business to produce goods and services.

Name:	us-gaap_CapitalLeasedAssetsGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

Name:	us-gaap_CapitalLeasedAssetsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The total gross amount less the charge for the use of the long-lived depreciable assets subject to a lease meeting the criteria for capitalization.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

+ Details

Name:	us-gaap_CapitalLeasesBalanceSheetAssetsByMajorClassNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of minimum lease payments for capital leases.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsDue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of minimum lease payments for capital leases due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsDueCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum lease payments for capital leases due in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsDueInThreeYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum lease payments for capital leases due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsDueInTwoYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount necessary to reduce net minimum lease payments to present value for capital leases.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsInterestIncludedInPayments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of minimum lease payments for capital leases net of executory costs, which include amounts paid by the lessee to the lessor for insurance, maintenance and taxes.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsNetMinimumPayments1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Present value of minimum lease payments for capital leases net of executory costs, including amounts paid by the lessee to the lessor for insurance, maintenance and taxes.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsPresentValueOfNetMinimumPayments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The total charge for the use of long-lived depreciable assets subject to a lease meeting the criteria for capitalization.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6455314&loc=d3e45014-112735

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 10
-Section 55
-Paragraph 40
-Subparagraph (Note 2)
-URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697

+ Details

Name:	us-gaap_CapitalLeasesLesseeBalanceSheetAssetsByMajorClassAccumulatedDeprecation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_ComputerEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_SoftwareAndSoftwareDevelopmentCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: