FORRESTER RESEARCH, INC. - 10-K filing on 2017-03-10

Stockholders' Equity - Additional Information (Detail)	3 Months Ended												12 Months Ended
Stockholders' Equity - Additional Information (Detail)	Dec. 31, 2016 USD ($) $ / shares shares	Sep. 30, 2016 $ / shares	Jun. 30, 2016 $ / shares	Mar. 31, 2016 $ / shares	Dec. 31, 2015 $ / shares shares	Sep. 30, 2015 $ / shares	Jun. 30, 2015 $ / shares	Mar. 31, 2015 $ / shares	Dec. 31, 2014 $ / shares	Sep. 30, 2014 $ / shares	Jun. 30, 2014 $ / shares	Mar. 31, 2014 $ / shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) Installment $ / shares	Dec. 31, 2012 shares	Oct. 31, 2016 USD ($)	Jul. 31, 2015 USD ($)	Feb. 28, 2015 USD ($)	Dec. 31, 2011 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred stock, shares authorized	500,000				500,000								500,000	500,000
Preferred stock, par value \| $ / shares	$ 0.01				$ 0.01								$ 0.01	$ 0.01
Authorized to purchase of common stock under the stock repurchase program \| $	$ 485,000,000												$ 485,000,000				$ 25,000,000	$ 25,000,000	$ 25,000,000
Cumulative repurchase of common stock	15,000,000												15,000,000
Aggregate cost of repurchase of common stock \| $													$ 424,900,000
Dividend declared and paid per share \| $ / shares	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.16	$ 0.16	$ 0.16	$ 0.16
Dividend declared per share \| $ / shares													$ 0.72	$ 0.68	$ 0.64
Aggregate dividend declared for the year \| $													$ 13,000,000	$ 12,200,000	$ 12,000,000
Option Outstanding and vested	1,540,000				2,171,000								1,540,000	2,171,000
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Future awards granted or issued under plans	200,000												200,000
Shares authorized	700,000												700,000
Hours of work per week													20 hours
Duration of purchase periods under employee stock purchase plan													6 months
Holding period of stocks acquired under employee stock purchase plan													1 year
Value used to derive maximum number of shares per participant for employee stock purchase \| $													$ 12,500
Employee maximum elected percentage reduction of compensation to purchase shares													10.00%
Exercise price rate of fair value													85.00%
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value for RSUs granted \| $ / shares													$ 37.87	$ 31.50	$ 36.67
Number of shares received per restricted stock unit on lapse of restrictions and vesting condition met													1
Value of RSUs vested and converted to common stock \| $													$ 6,600,000	$ 4,600,000	$ 3,500,000
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of options exercised \| $													$ 3,700,000	$ 400,000	$ 2,300,000
Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock options entitlement of non- employee directors	12,000												12,000
Number of installments upon which share based awards vest \| Installment															4
1996 Equity Plan [Member] \| Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Future awards granted or issued under plans	0												0
2006 Director Plan [Member] \| Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Future awards granted or issued under plans	0												0
Shares authorized	450,000,000												450,000,000
Shares returned																80,000
Number of shares of common stock options entitlement of non- employee directors																				6,000,000
Option Outstanding and vested	200,000												200,000
1996 Director [Member] \| Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Future awards granted or issued under plans	0												0
Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity incentive plan extended term													10 years
Equity incentive plan expiration													2026-05
Increase in number of shares issuable under plan													2,000,000
Equity Incentive Plan [Member] \| Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Future awards granted or issued under plans	2,700,000												2,700,000
Shares authorized																6,350,000
Shares returned																2,500,000
Option expiration period													10 years
Equity Incentive Plan [Member] \| Equity Plan [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options vested period													4 years
Prior to 2011 [Member] \| Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock options entitlement of non- employee directors	12,500												12,500