Business combinations - Presidio (Details)		1 Months Ended	9 Months Ended
	Oct. 11, 2019 USD ($) $ / shares shares	Oct. 31, 2019	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)
Liabilities assumed:
Subordinate debt			$ 39,693,000	$ 39,554,000
Goodwill recorded in the merger			167,631,000	$ 167,420,000	$ 83,753,000
Aggregate transaction value	$ 185,598,000
Issuance of shares of common stock to Presidio shareholders | shares	15,684,064
Issuance of shares of common stock to holders of restricted stock	$ 178,171,000
Stock price at Closing (in dollars per share) | $ / shares	$ 11.36
Consideration for Presidio stock options exchanged for Heritage Commerce Corp stock options	$ 7,426,000
Cash paid for fractional shares	1,000
Presidio bank
Assets acquired:
Cash and cash equivalents	117,988,000
Securities available-for-sale	45,069,000
Securities held-to-maturity	463,000
Loans	685,964,000
Premises and equipment	1,756,000
Other intangible assets	11,147,000
Other assets, net	42,161,000
Total assets acquired	904,548,000
Liabilities assumed:
Deposits	774,259,000
Subordinate debt	10,000,000
Other borrowings	442,000
Other liabilities	18,127,000
Total liabilities assumed	802,828,000
Net assets acquired	101,720,000
Purchase price	185,598,000
Goodwill recorded in the merger	83,878,000
Aggregate transaction value	$ 185,598,000
Fixed exchange ratio	2.47
Issuance of shares of common stock to Presidio shareholders | shares	15,684,064
Issuance of shares of common stock to holders of restricted stock	$ 178,171,000
Stock price at Closing (in dollars per share) | $ / shares	$ 11.36
Consideration for Presidio stock options exchanged for Heritage Commerce Corp stock options	$ 7,426,000		$ 7,426,000
Cash paid for fractional shares	1,000
Amortized intangible assets		60 months
Previously Reported | Presidio bank
Assets acquired:
Cash and cash equivalents	117,989,000
Securities available-for-sale	44,647,000
Securities held-to-maturity	463,000
Loans	698,493,000
Allowance for loan losses	(7,463,000)
Premises and equipment	1,756,000
Other assets, net	43,539,000
Total assets acquired	899,424,000
Liabilities assumed:
Deposits	774,260,000
Subordinate debt	10,000,000
Other borrowings	442,000
Other liabilities	17,916,000
Total liabilities assumed	802,618,000
Fair Value Adjustments | Presidio bank
Assets acquired:
Cash and cash equivalents	(1,000)
Securities available-for-sale	422,000
Loans	(12,529,000)
Allowance for loan losses	7,463,000
Other intangible assets	11,147,000
Other assets, net	(1,378,000)
Total assets acquired	5,124,000
Liabilities assumed:
Deposits	(1,000)
Other liabilities	211,000
Total liabilities assumed	$ 210,000