Stockholders' Equity - Additional Information (Detail) - USD ($)		1 Months Ended		12 Months Ended
	Nov. 30, 2016	Mar. 31, 2017	Mar. 31, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Authorized capital stock				520,000,000
Common stock, par value				$ 0.0001	$ 0.0001
Common stock, shares authorized				500,000,000	500,000,000
Preferred stock, par value				$ 0.0001
Preferred stock, shares authorized				20,000,000
Preferred stock, shares outstanding				0	0
Common stock, shares issued				30,730,000	28,933,000
Stock-based compensation expense				$ 23,100,000	$ 16,784,000	$ 10,251,000
Stock option granted description terms				the exercise price of an option granted to a 10% shareholder cannot be less than 110% of the estimated fair value of the shares on the date of grant. Options granted to employees and non-employees generally vest over four years and expire in seven years.
Ownership percent				10.00%
Shares of common stock ,reserved for issuance				11,278,843
Aggregate intrinsic value				$ 7,433,000
Common stock, shares exercised				60,125	18,947	23,822
Option intrinsic value, exercised				$ 200,000	$ 200,000	$ 600,000
Net tax benefits related to exercised options				0
Estimated fair value of stock option, vested				14,000,000	14,000,000	$ 2,900,000
Stock may be issued or transferred, value				$ 1,688,000	600,000
Granted, Number of shares				1,694,000
Weighted average exercise price of options granted				$ 15.79
2014 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Annual increase in EIP/ESPP, subject to other limitations				5.00%
Shares of common stock ,reserved for issuance				10,214,174
Aggregate number of awards available for grant to be issued				3,557,041
Outstanding options and RSUs				6,657,133
2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense				$ 600,000	$ 400,000
Percentage of employees purchase price of common stock				85.00%
Shares of common stock ,reserved for issuance				789,669
Aggregate number of awards available for grant to be issued				789,669
Unrecognized stock-based compensation expense				$ 200,000
Employee stock purchase plan effective date				Oct. 15, 2014
Employee stock purchase plan description				(i) the beginning of a 12-month offering period, or (ii) at the end of one of the two related 6-month purchase periods
Employee stock purchase plan offering commenced start date				Jun. 01, 2016
Shares purchased	22,844			81,922
Offering period commenced date				Jun. 01, 2017
Offering period end date				May 31, 2018
Outside 2014 EIP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock ,reserved for issuance				275,000
Unrecognized stock-based compensation expense				$ 2,000,000
Aggregate intrinsic value				$ 1,100,000
Unrecognized stock-based compensation weighted average recognition period				3 years 6 months
Granted, Number of shares				275,000	0	0
Weighted average exercise price of options granted				$ 13.96
Weighted average grant date fair value of options granted				$ 2,200,000
Unvested RSAs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Combined grant date intrinsic value for award							$ 1,700,000
Shares, Granted				0	0	0
Unvested RSAs | Service And Performance Conditions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense				$ 0	$ 200,000	$ 800,000
Unvested RSAs | 2012 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Proceeds from sale of common stock, net			$ 300,000
Unvested RSAs | Chief Executive Officer
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued							1,066,154
Unvested RSAs | Atara Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued			269,230
Unvested RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Granted				782,413
Weighted average grant date fair value per share				$ 15.07	$ 17.83	$ 25.15
Unrecognized stock-based compensation expense				$ 20,600,000
Aggregate intrinsic value				$ 30,800,000
Unrecognized stock-based compensation weighted average recognition period				2 years 7 months 6 days
Restricted stock units, settled				327,282	204,611
Restricted stock units, issued net of tax withholdings				52,624	13,573
Restricted stock units withheld for tax obligations				22,274	5,222
Restricted stock units withheld for tax obligations, value				$ 400,000	$ 100,000
Unvested RSUs | From Date Of Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period				7 years
Unvested RSUs | Following Service Termination Date
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period				2 years
Employees And Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period				7 years
Share based compensation, vesting period				4 years
Vested and Unvested Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation weighted average recognition period				2 years 7 months 6 days
Unrecognized stock-based compensation				$ 30,300,000
Maximum | 2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Annual increase in EIP/ESPP, subject to other limitations				1.00%
Maximum increase in number of shares available for issuance				230,769
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of employees purchase price of common stock				100.00%
Minimum | 10% Shareholder
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of employees purchase price of common stock				110.00%
Minimum | 2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock may be issued or transferred, value				$ 25,000
At The Market Offering
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Proceeds from sale of common stock, net				$ 19,156,000	$ 0	$ 0
At The Market Offering | Cowen
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued				1,349,865
Common stock average price				$ 14.82
Proceeds from sale of common stock, gross				$ 20,000,000
Proceeds from sale of common stock, net				19,200,000
Common stock value remained available to be sold				$ 55,000,000
At The Market Offering | Cowen | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock aggregate offering price		$ 75,000,000
Percentage of commission to be paid on gross sales proceeds of common stock sold		3.00%