Stock-Based Compensation - Additional Information (Details)		1 Months Ended				12 Months Ended
	Nov. 07, 2018 USD ($) solar_energy_system_install	Aug. 31, 2017 USD ($) solar_energy_system_install $ / shares shares	Aug. 31, 2015 USD ($) shares	Jul. 31, 2015 shares	Mar. 31, 2015 shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2014 shares	Aug. 31, 2014 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares outstanding (in shares)						107,350,000	104,321,000
Common stock reserved for issuance (in shares)						34,960,000	34,425,000
Unvested exercisable shares						276,660	276,660
Unvested options exercised						0	0
Weighted-average grant-date fair value of stock options granted | $ / shares						$ 4.21	$ 2.57	$ 2.26
Total intrinsic value of options exercised | $						$ 21,400,000	$ 2,300,000	$ 6,300,000
Total fair value of options vested | $						8,800,000	$ 7,500,000	9,800,000
Unvested options subject to repurchase, outstanding							0
Compensation expense recognized | $						27,856,000	$ 22,042,000	18,723,000
Total unrecognized compensation cost | $						$ 40,000,000	$ 34,800,000
Weighted-average period of recognition						2 years 3 months 18 days	2 years 9 months 12 days
Warrants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrant, term		40 months
Warrant, number purchased (in shares)		11,793,355
Warrant, exercise price (in dollars per share) | $ / shares		$ 0.01
Warrant, vesting percentage		50.05%
Warrant, milestone one, solar energy systems | solar_energy_system_install	6,000	30,000
Warrant, milestone two, marketing and sales dollars | $		$ 10,000,000
Warrant, vesting term		5 years
Warrant, milestone two, solar energy systems | solar_energy_system_install	8,000	6,000
Warrant, vesting percentage, milestone one	10.00%
Warrant, vesting percentage, milestone two	13.30%
Warrant, milestone one and two, marketing and sales dollars | $	$ 25,000,000.0
Warrant, vesting percentage, milestone three	8.30%
Warrant, milestone three, solar energy systems | solar_energy_system_install	5,000
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)						4,182,000	5,330,000
Stock option plan, number of shares granted (in shares)						2,019,000	3,540,000		947,342
Vesting starting period						2014-08
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)						13,590,000	16,268,000
Performance-based Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense recognized | $						$ 3,500,000	$ 800,000	$ 1,200,000
Expire in August 2017 | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period of RSUs						2017-08
Expire in September 2017 | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period of RSUs						2017-09
Expire in September 2019 | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period of RSUs						2019-09
MEC 2009 Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares outstanding (in shares)						262,971
2013 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional common stock shares reserved for issuance (in shares)					3,000,000
Common stock reserved for issuance (in shares)					4,500,000
Stock options vesting period					4 years
Stock option plan, number of shares granted (in shares)						0
2013 Equity Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)					8,044,829
Stock options granted, expiration period					10 years
2013 Equity Incentive Plan | End of One Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting award percentage					25.00%
2013 Equity Incentive Plan | Monthly Over the Remaining Three Years
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting award percentage					75.00%
2014 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)				1,197,342						947,342
2014 Equity Incentive Plan | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option plan, number of shares granted (in shares)				1,197,342
2015 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)				11,400,000		11,986,000	8,857,000
Stock options vesting period				4 years
Minimum annual automatic increase included in common stock reserved for future issuance as of first day of each fiscal year				10,000,000
Minimum percentage of annual automatic increase included in common stock reserved for future issuance as of last day of immediately preceding fiscal year				4.00%
Additional common stock shares reserved for issuance						4,294,010	4,172,883
Terms of shares available for issuance						The automatic increase of the number of shares available for issuance under the 2015 Plan is equal to the least of 10 million shares, 4% of the outstanding shares of common stock as of the last day of our immediately preceding fiscal year or such other amount as the Board of Directors may determine.
2015 Equity Incentive Plan | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period				4 years
Outstanding RSUs						350,000
2015 Equity Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)				15,439,334
Stock options granted, expiration period				10 years
2015 Equity Incentive Plan | End of One Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting award percentage				25.00%
2015 Equity Incentive Plan | End of One Year | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting award percentage				25.00%
2015 Equity Incentive Plan | Monthly Over the Remaining Three Years
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting award percentage				75.00%
2015 Equity Incentive Plan | Quarterly Over the Remaining Three Years | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting award percentage				75.00%
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)			1,000,000
Minimum percentage of annual automatic increase included in common stock reserved for future issuance as of last day of immediately preceding fiscal year			2.00%
Additional common stock shares reserved for issuance						2,147,005	2,086,416
Annual automatic increase included in common stock reserved for future issuance as of last day of immediately preceding fiscal year | $			$ 5,000,000
Terms of shares available for issuance						The ESPP provides for an automatic increase of the number of shares available for issuance under the ESPP on the first day of each fiscal year beginning on January 1, 2016, equal to the least of 5 million shares, 2% of the outstanding shares of our common stock on the last day of the immediately preceding fiscal year, or such other amount as may be determined by the Board of Directors.
Offering period						24 months
Maximum percentage in payroll deductions to acquire shares of common stock			15.00%
Maximum deductible fair market value of shares available for employee to purchase per calendar year | $			$ 25,000
Maximum number of shares available for employee to purchase per offering period			10,000
Term of offering period						Eligible employees are offered shares bi-annually through two six month offering periods, which begin on the first trading day on or after May 15 and November 15 of each year.