RELATED PARTIES - Series G / Series G’ (Details) - $ / shares	1 Months Ended
	May 31, 2017	Mar. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Shares outstanding (in shares)		20,226,000	3,439,000
Class of warrant, numerator input value (in usd per share)	$ 68.9516
Number of trading days	30 days
Series G Stock
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0	346,000
Class of warrant, numerator input value (in usd per share)	$ 68.9516
Number of trading days	30 days
Series G’ Stock
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0	1,295,000
Aimia Inc. | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		801,000
Aimia Inc. | Series G Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
Aimia Inc. | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		382,000
Polaris Venture Partners | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
Polaris Venture Partners | Series G Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		29,000
Polaris Venture Partners | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		212,000
Canaan VIII L.P. | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
Canaan VIII L.P. | Series G Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		54,000
Canaan VIII L.P. | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		260,000
Discovery Capital | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
Discovery Capital | Series G Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
Discovery Capital | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		106,000
Scott D. Grimes | Scott D. Grimes
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
Scott D. Grimes | Series G Stock | Scott D. Grimes
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
Scott D. Grimes | Series G’ Stock | Scott D. Grimes
Related Party Transaction [Line Items]
Shares outstanding (in shares)		26,000
Lynne M. Laube | Lynne M. Laube
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
Lynne M. Laube | Series G Stock | Lynne M. Laube
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
Lynne M. Laube | Series G’ Stock | Lynne M. Laube
Related Party Transaction [Line Items]
Shares outstanding (in shares)		14,000
Entities affiliated with Mark A. Johnson | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
Entities affiliated with Mark A. Johnson | Series G Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		35,000
Entities affiliated with Mark A. Johnson | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		15,000
Mark A. Johnson | Series G Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		5,801
John Klinck | Director
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
John Klinck | Series G Stock | Director
Related Party Transaction [Line Items]
Shares outstanding (in shares)		6,000
John Klinck | Series G’ Stock | Director
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
David Adams | Director
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
David Adams | Series G Stock | Director
Related Party Transaction [Line Items]
Shares outstanding (in shares)		3,000
David Adams | Series G’ Stock | Director
Related Party Transaction [Line Items]
Shares outstanding (in shares)		0
Aeroplan Holdings Europe Sàrl | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		159,207
Aimia EMEA Limited | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		801,329
Aimia EMEA Limited | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		223,020
Polaris Venture Partners V, L.P. | Series G Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		27,988
Polaris Venture Partners V, L.P. | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		205,020
Polaris Venture Partners Entrepreneurs’ Fund V, L.L. | Series G Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		545
Polaris Venture Partners Entrepreneurs’ Fund V, L.L. | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		3,995
Polaris Venture Partners Founders’ Fund V, L.P. | Series G Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		191
Polaris Venture Partners Founders’ Fund V, L.P. | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		1,404
Polaris Venture Partners Special Founders’ Fund V, L.P. | Series G Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		280
Polaris Venture Partners Special Founders’ Fund V, L.P. | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		2,050
Discovery Opportunity Master Fund, Ltd. | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		95,272
Discovery Global Opportunity Master Fund, Ltd. | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		11,072
TTP Fund II, L.P. | Series G’ Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		15,045
TTV Ivy Holdings, LLC | Series G Stock | Affiliated Entity
Related Party Transaction [Line Items]
Shares outstanding (in shares)		29,005