Property and equipment, net - Narrative (Details) - Land and Two Buildings in Wilmington, Delaware $ in Millions	1 Months Ended
Mar. 31, 2017 USD ($)	Sep. 30, 2016 building agreement
Property and equipment, net
Purchase price of land and office building(s)	$ 8.1
Number of agreements \| agreement		2
Number of buildings \| building		2
Land
Property and equipment, net
Purchase price of land and office building(s)	1.2
Building And Leasehold Improvements
Property and equipment, net
Purchase price of land and office building(s)	$ 6.9

The cash outflow from the acquisition of a piece of land, anything permanently fixed to it, including buildings, structures on it and so forth; includes real estate intended to generate income for the owner; excludes real estate acquired for use by the owner.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3213-108585

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration