Outstanding Loans and Security Agreements - Recourse Debt Facilities (Additional Information) (Details)			1 Months Ended
	Jul. 01, 2017	May 22, 2013 USD ($)	Dec. 31, 2014 USD ($) related_party shares	Jun. 30, 2018 USD ($) $ / shares	Jan. 18, 2018	Dec. 31, 2017 USD ($)	Aug. 31, 2017 shares	Jun. 30, 2017 USD ($) agreement	Dec. 31, 2016 shares	Sep. 30, 2016 USD ($) agreement	Sep. 20, 2016 USD ($)	Sep. 10, 2016 USD ($)	Jan. 29, 2016 USD ($)	Dec. 15, 2015 USD ($) $ / shares	Jun. 30, 2015 USD ($)	Feb. 28, 2015 USD ($) agreement
Debt Instrument [Line Items]
Long-term debt carrying value				$ 960,129,000		$ 941,341,000
Interest rate percentage						8.00%
Unpaid principal balance				1,016,277,000		$ 1,008,104,000
Long-term portion of debt				$ 930,123,000		$ 921,205,000
Notes due July 2024, Recourse
Debt Instrument [Line Items]
Interest rate percentage				10.00%		10.00%
Term loan | Term Loan due November 2020, Recourse
Debt Instrument [Line Items]
Debt face amount		$ 12,000,000
Debt term		90 months
Weighted average interest rate (as a percentage)						5.10%
Long-term debt carrying value				$ 4,050,000		$ 4,887,000
Unpaid principal balance				4,050,000		5,000,000
Long-term portion of debt				2,362,000		3,197,000
Convertible promissory notes | Convertible Promissory Notes due December 2019 and 2020, Recourse
Debt Instrument [Line Items]
Debt term			3 years
Long-term debt carrying value				$ 254,120,000		$ 244,717,000
Interest rate percentage			8.00%	5.00%	5.00%	8.00%
Number of related parties | related_party			3
Unpaid principal balance				$ 254,120,000		$ 244,717,000
Long-term portion of debt				245,457,000		244,717,000
Convertible promissory notes | Convertible Promissory Notes, January and February 2015, Recoursee
Debt Instrument [Line Items]
Debt face amount																$ 34,000,000
Number of agreements | agreement																2
Convertible promissory notes | Convertible Promissory Notes, June 2015, Recourse
Debt Instrument [Line Items]
Debt face amount															$ 27,000,000
Convertible promissory notes | Convertible Promissory Notes due December 2020, Recourse
Debt Instrument [Line Items]
Debt face amount											$ 75,000,000	$ 75,000,000	$ 25,000,000	$ 160,000,000
Long-term debt carrying value				$ 254,062,000		236,724,000
Interest rate percentage	6.00%			6.00%				5.00%
Convertible stock price (in dollars per share) | $ / shares				$ 46.37										$ 46.37
Unpaid principal balance				$ 294,759,000		286,069,000
Long-term portion of debt				254,062,000		236,724,000
Notes | Notes due July 2024, Recourse
Debt Instrument [Line Items]
Debt face amount								$ 100,000,000
Long-term debt carrying value				95,140,000		94,517,000
Interest rate percentage								10.00%
Unpaid principal balance				100,000,000		100,000,000
Long-term portion of debt				95,140,000		94,517,000
Number of agreements | agreement								5
Convertible redeemable common stock warrants
Debt Instrument [Line Items]
Number of securities called by warrants (in shares) | shares			33,333
Affiliated entity
Debt Instrument [Line Items]
Long-term debt carrying value				$ 108,800,000		107,000,000
Affiliated entity | Convertible promissory notes | Convertible Promissory Notes due December 2019 and 2020, Recourse
Debt Instrument [Line Items]
Debt face amount			$ 132,200,000
Debt term			3 years
Interest rate percentage			8.00%
Convertible stock price (in dollars per share) | $ / shares				$ 38.64
Unpaid principal balance				$ 39,900,000		38,300,000
Affiliated entity | Convertible promissory notes | Convertible Promissory Notes due December 2020, Recourse
Debt Instrument [Line Items]
Debt face amount										$ 260,000,000
Interest rate percentage	6.00%						6.00%		6.00%	5.00%
Unpaid principal balance				$ 27,600,000		$ 26,800,000
Number of agreements | agreement										6
Debt interest increase (as a percentage)	1.00%
Affiliated entity | Alberta Investment Management Corporation | Convertible promissory notes | Convertible Promissory Notes due December 2019 and 2020, Recourse
Debt Instrument [Line Items]
Debt face amount			$ 10,000,000
Affiliated entity | KPCB Holdings, Inc | Convertible promissory notes | Convertible Promissory Notes due December 2019 and 2020, Recourse
Debt Instrument [Line Items]
Debt face amount			10,000,000
Affiliated entity | New Enterprise Associates | Convertible promissory notes | Convertible Promissory Notes due December 2019 and 2020, Recourse
Debt Instrument [Line Items]
Debt face amount			$ 10,000,000
Affiliated entity | Convertible redeemable common stock warrants | CPPIB
Debt Instrument [Line Items]
Number of securities called by warrants (in shares) | shares							312,888
Maximum | Convertible redeemable common stock warrants | J.P. Morgan
Debt Instrument [Line Items]
Number of securities called by warrants (in shares) | shares									146,666
Maximum | Convertible redeemable common stock warrants | CPPIB
Debt Instrument [Line Items]
Number of securities called by warrants (in shares) | shares									166,222
Maximum | Affiliated entity | Convertible redeemable common stock warrants | J.P. Morgan
Debt Instrument [Line Items]
Number of securities called by warrants (in shares) | shares									146,666
Maximum | Affiliated entity | Convertible redeemable common stock warrants | CPPIB
Debt Instrument [Line Items]
Number of securities called by warrants (in shares) | shares									166,222