SUBSEQUENT EVENTS	12 Months Ended
Dec. 31, 2019
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	SUBSEQUENT EVENTS Listing Non-Compliance Notice On March 2, 2020, the Company received written notice (the “Notice”) from Nasdaq Stock Market LLC ("Nasdaq") indicating that the Company is not in compliance with the $1.00 minimum bid price requirement for continued listing on the Nasdaq Global Market, as set forth in Nasdaq Listing Rule 5450(a)(1). The Notice has no effect at this time on the listing of the Company’s common stock, which will continue to trade on the Nasdaq Global Market under the symbol “SESN.” In accordance with Nasdaq Listing Rule 5810(c)(3)(A), the Company has a period of 180 calendar days, or until August 31, 2020, to regain compliance with the minimum bid price requirement. To regain compliance, the closing bid price of the Company’s common stock must meet or exceed $1.00 per share for a minimum of ten consecutive business days during this 180-day period. If the Company is not in compliance by August 31, 2020, the Company may be afforded a second 180 calendar day period to regain compliance. To qualify, the Company would be required to apply to have its common stock listed on the Nasdaq Capital Market and meet the continued listing requirement for market value of publicly held shares and all other initial listing standards for the Nasdaq Capital Market, except for the minimum bid price requirement, and will need to provide written notice to Nasdaq of its intention to cure the deficiency during the second compliance period by effecting a reverse stock split, if necessary. The Company intends to monitor the closing bid price of its common stock and may, if appropriate, consider implementing available options to regain compliance with the minimum bid price requirement, including, but not limited to, a proposal for a reverse stock split of the Company's common stock to be submitted for stockholder approval at the Company's 2020 Annual Meeting of Stockholders. Stock Options Granted In January 2020, the Company granted 0.1 million stock options with an exercise price of $0.84 to new employees as inducement equity awards outside of the Company's 2014 Plan in accordance with Nasdaq Listing Rule 5635(c)(4). These stock options had a grant-date fair value of $0.59 and vest at the rate of 25% of the grant on the first anniversary of the date of grant and thereafter 6.25% of the grant at the end of each successive three-month period following the first anniversary of the date of grant, subject to the recipient's continued service with the Company through the applicable vesting dates. In February 2020, the Company's board of directors granted 3.1 million stock options with an exercise price of $0.89 to employees and officers under the Company's 2014 Plan. These stock options had a grant-date fair value of $0.62 and vest at the rate of 6.25% of the grant every three months over a period of four years, subject to the recipient's continued service with the Company through the applicable vesting dates.

SUBSEQUENT EVENTS

Listing Non-Compliance Notice

On March 2, 2020, the Company received written notice (the “Notice”) from Nasdaq Stock Market LLC ("Nasdaq") indicating that the Company is not in compliance with the $1.00 minimum bid price requirement for continued listing on the Nasdaq Global Market, as set forth in Nasdaq Listing Rule 5450(a)(1). The Notice has no effect at this time on the listing of the Company’s common stock, which will continue to trade on the Nasdaq Global Market under the symbol “SESN.”

In accordance with Nasdaq Listing Rule 5810(c)(3)(A), the Company has a period of 180 calendar days, or until August 31, 2020, to regain compliance with the minimum bid price requirement. To regain compliance, the closing bid price of the Company’s common stock must meet or exceed $1.00 per share for a minimum of ten consecutive business days during this 180-day period. If the Company is not in compliance by August 31, 2020, the Company may be afforded a second 180 calendar day period to regain compliance. To qualify, the Company would be required to apply to have its common stock listed on the Nasdaq Capital Market and meet the continued listing requirement for market value of publicly held shares and all other initial listing standards for the Nasdaq Capital Market, except for the minimum bid price requirement, and will need to provide written notice to Nasdaq of its intention to cure the deficiency during the second compliance period by effecting a reverse stock split, if necessary.

The Company intends to monitor the closing bid price of its common stock and may, if appropriate, consider implementing available options to regain compliance with the minimum bid price requirement, including, but not limited to, a proposal for a reverse stock split of the Company's common stock to be submitted for stockholder approval at the Company's 2020 Annual Meeting of Stockholders.

Stock Options Granted

In January 2020, the Company granted 0.1 million stock options with an exercise price of $0.84 to new employees as inducement equity awards outside of the Company's 2014 Plan in accordance with Nasdaq Listing Rule 5635(c)(4). These stock options had a grant-date fair value of $0.59 and vest at the rate of 25% of the grant on the first anniversary of the date of grant and thereafter 6.25% of the grant at the end of each successive three-month period following the first anniversary of the date of grant, subject to the recipient's continued service with the Company through the applicable vesting dates.

In February 2020, the Company's board of directors granted 3.1 million stock options with an exercise price of $0.89 to employees and officers under the Company's 2014 Plan. These stock options had a grant-date fair value of $0.62 and vest at the rate of 6.25% of the grant every three months over a period of four years, subject to the recipient's continued service with the Company through the applicable vesting dates.