Stock-based Compensation - Summary of activity (Details) - Class D	12 Months Ended
Dec. 31, 2017 shares
Summary of the activity in the units
Balances at beginning of the year (in units)	201,696
Vested converted to D units (in units)	(48,939)
Cancelled unvested (in units)	(152,757)

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration