Redeemable Convertible Preferred Stock - Schedule of Redeemable Preferred Stock (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Temporary Equity [Line Items]
Preferred Shares Authorized	0	151,557,293
Preferred Shares Issued	0	105,631,019
Preferred Shares Outstanding	0	105,631,019	81,975,780	81,975,780
Carrying Value	$ 0	$ 129,745	$ 97,681	$ 97,610
Liquidation Preference		$ 130,053
Common Stock Issuable Upon Conversion		10,461,117
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		1,250,000
Preferred Shares Issued		1,250,000
Preferred Shares Outstanding		1,250,000
Carrying Value		$ 1,250
Liquidation Preference		$ 1,250
Common Stock Issuable Upon Conversion		12,579
Series A-1 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		615,384
Preferred Shares Issued		615,384
Preferred Shares Outstanding		615,384
Carrying Value		$ 800
Liquidation Preference		$ 800
Common Stock Issuable Upon Conversion		6,192
Series B Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		3,706,056
Preferred Shares Issued		3,706,056
Preferred Shares Outstanding		3,706,056
Carrying Value		$ 1,506
Liquidation Preference		$ 1,506
Common Stock Issuable Upon Conversion		372,955
Series C-1 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		5,934,050
Preferred Shares Issued		5,934,050
Preferred Shares Outstanding		5,934,050
Carrying Value		$ 6,997
Liquidation Preference		$ 7,000
Common Stock Issuable Upon Conversion		597,167
Series C-2 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		8,689,144
Preferred Shares Issued		8,689,144
Preferred Shares Outstanding		8,689,144
Carrying Value		$ 10,248
Liquidation Preference		$ 10,250
Common Stock Issuable Upon Conversion		874,423
Series D Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		34,142,865
Preferred Shares Issued		34,142,865
Preferred Shares Outstanding		34,142,865
Carrying Value		$ 40,263
Liquidation Preference		$ 40,276
Common Stock Issuable Upon Conversion		3,435,932
Series D-1 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		363,636
Preferred Shares Issued		363,636
Preferred Shares Outstanding		363,636
Carrying Value		$ 2,000
Liquidation Preference		$ 2,000
Common Stock Issuable Upon Conversion		36,594
Series E Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		12,715,822
Preferred Shares Issued		3,788,240
Preferred Shares Outstanding		3,788,240
Carrying Value		$ 4,453
Liquidation Preference		$ 4,469
Common Stock Issuable Upon Conversion		381,225
Series E-1 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		24,264,705
Preferred Shares Issued		7,697,597
Preferred Shares Outstanding		7,697,597
Carrying Value		$ 10,446
Liquidation Preference		$ 10,469
Common Stock Issuable Upon Conversion		774,639
Series E-2 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		9,226,082
Preferred Shares Issued		8,927,582
Preferred Shares Outstanding		8,927,582
Carrying Value		$ 10,493
Liquidation Preference		$ 10,531
Common Stock Issuable Upon Conversion		898,418
Series E-3 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		21,237,785
Preferred Shares Issued		16,567,108
Preferred Shares Outstanding		16,567,108
Carrying Value		$ 22,483
Liquidation Preference		$ 22,531
Common Stock Issuable Upon Conversion		1,667,214
Series F Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		29,411,764
Preferred Shares Issued		13,949,357
Preferred Shares Outstanding		13,949,357
Carrying Value		$ 18,806
Liquidation Preference		$ 18,971
Common Stock Issuable Upon Conversion		1,403,779