Related Party Transactions - Additional Information (Details)			1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 31, 2017 USD ($) shares	Jul. 01, 2017	Jul. 31, 2018 shares	Dec. 31, 2014 USD ($) shares	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) shares	Dec. 31, 2013 USD ($)	Jul. 27, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Jan. 18, 2018	Sep. 30, 2016 USD ($) agreement	Sep. 20, 2016 USD ($)	Sep. 10, 2016 USD ($)	Jan. 29, 2016 USD ($)	Dec. 15, 2015 USD ($) $ / shares	Dec. 31, 2012 USD ($)
Related Party Transaction [Line Items]
Long-term debt carrying value					$ 740,811,000		$ 740,811,000		$ 941,341,000
Unpaid principal balance					792,713,000		792,713,000		1,008,104,000
Interest paid							$ 30,601,000	$ 16,874,000
Issuance of common stock upon exercise of warrants (in shares) | shares							312,575
Issuance of common stock warrant						$ 9,410,000		9,410,000
Affiliated entity
Related Party Transaction [Line Items]
Long-term debt carrying value					68,700,000		$ 68,700,000		107,000,000
Issuance of common stock warrant	$ 9,400,000
Consulting Agreement | General Colin L. Powell | Director
Related Party Transaction [Line Items]
Related party amount of transaction							125,000
Convertible promissory notes | Convertible Promissory Notes due December 2019 and 2020, Recourse
Related Party Transaction [Line Items]
Long-term debt carrying value					$ 34,273,000		$ 34,273,000
Debt term				3 years
Interest rate percentage					5.00%		5.00%							5.00%
Unpaid principal balance					$ 34,273,000		$ 34,273,000					$ 221,600,000
Convertible promissory notes | Convertible Promissory Notes due December 2019 and 2020, Recourse | Affiliated entity
Related Party Transaction [Line Items]
Debt term				3 years
Debt face amount				$ 132,200,000
Interest rate percentage				8.00%
Convertible stock price (in dollars per share) | $ / shares					$ 38.64		$ 38.64					$ 38.64
Unpaid principal balance									38,300,000				$ 39,900,000
Convertible promissory notes | Convertible Promissory Notes due December 2019 and 2020, Recourse | Alberta Investment Management Corporation | Affiliated entity
Related Party Transaction [Line Items]
Debt face amount				$ 10,000,000
Convertible promissory notes | Convertible Promissory Notes due December 2020, Recourse
Related Party Transaction [Line Items]
Long-term debt carrying value					$ 258,862,000		$ 258,862,000		$ 236,724,000
Debt face amount																$ 75,000,000	$ 75,000,000	$ 25,000,000	$ 160,000,000
Interest rate percentage		6.00%			6.00%		6.00%		6.00%
Convertible stock price (in dollars per share) | $ / shares					$ 46.37		$ 46.37												$ 11.25
Unpaid principal balance					$ 296,233,000		$ 296,233,000		$ 286,069,000
Convertible promissory notes | Convertible Promissory Notes due December 2020, Recourse | Affiliated entity
Related Party Transaction [Line Items]
Debt face amount															$ 260,000,000
Interest rate percentage		6.00%								6.00%					5.00%
Unpaid principal balance					27,700,000		27,700,000		26,800,000
Number of agreements | agreement															5
Debt interest increase (as a percentage)		1.00%
PPA Company IIIa
Related Party Transaction [Line Items]
Repayments of debt							4,031,000		3,041,000
PPA Company IIIa | Term loan | Term Loan due September 2028, Non-Recourse
Related Party Transaction [Line Items]
Long-term debt carrying value					$ 36,502,000		$ 36,502,000		$ 36,940,000
Debt face amount																				$ 46,800,000
Interest rate percentage					7.50%		7.50%		7.50%											7.50%
Unpaid principal balance					$ 40,937,000		$ 40,937,000		$ 41,927,000
PPA Company IIIa | Term loan | Term Loan due September 2028, Non-Recourse | Affiliated entity
Related Party Transaction [Line Items]
Debt face amount				$ 45,000,000
Unpaid principal balance					40,900,000		40,900,000		41,900,000
Repayments of debt					0	200,000	600,000	1,300,000
Interest paid					$ 800,000	$ 800,000	$ 2,400,000	$ 4,800,000
Common stock warrants
Related Party Transaction [Line Items]
Number of securities called by warrants (in shares) | shares				33,333
Revaluation of warrants										$ 9,200,000	$ 3,300,000
Common stock warrants | Affiliated entity
Related Party Transaction [Line Items]
Revaluation of warrants									$ 200,000
Common stock warrants | CPPIB | Affiliated entity
Related Party Transaction [Line Items]
Number of securities called by warrants (in shares) | shares	312,888
Maximum | Common stock warrants | J.P. Morgan
Related Party Transaction [Line Items]
Number of securities called by warrants (in shares) | shares										146,666
Maximum | Common stock warrants | J.P. Morgan | Affiliated entity
Related Party Transaction [Line Items]
Number of securities called by warrants (in shares) | shares										146,666
Maximum | Common stock warrants | CPPIB
Related Party Transaction [Line Items]
Number of securities called by warrants (in shares) | shares										166,222
Maximum | Common stock warrants | CPPIB | Affiliated entity
Related Party Transaction [Line Items]
Number of securities called by warrants (in shares) | shares										166,222
Series G preferred
Related Party Transaction [Line Items]
Debt conversion, shares issued (in shares) | shares			1,038,050