Loans and reserve for credit losses (Narrative) (Details)			3 Months Ended
	Aug. 01, 2016 USD ($)	May 16, 2014 USD ($)	Mar. 31, 2017 USD ($) payment	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Financing Receivable, Impaired [Line Items]
Percentage of real estate related loans			78.40%
Loans participated to third-parties			$ 125,500,000		$ 128,800,000
Loans and leases receivable, before fees, gross, amount			$ 2,115,572,000		2,104,412,000
Percentage of loans acquired, substandard			3.30%
Loans and leases receivable, gross			$ 2,115,572,000		2,104,412,000
Percentage of acquired loans covered			7.00%
Loan loss (provision)/ recovery			$ 0	$ 0
Loans and leases receivable, allowance			25,357,000		25,290,000
Total accruing loans which are contractually past due 90 days or more			624,000		17,000
Impaired loans, total recorded balance			7,392,000		7,674,000
Interest income recognized for cash payments received on impaired loans			200,000	100,000
TDR outstanding recorded investment			0	22,000
Troubled Debt Restructuring
Financing Receivable, Impaired [Line Items]
Loan loss (provision)/ recovery			0	0
Loans and leases receivable, allowance			0		0
Impaired loans, total recorded balance			$ 0		0
Number of payments before upgrade to accrual status review | payment			6
Loans and leases receivable, impaired, commitment to lend			$ 0	0
TDR outstanding recorded investment			0	0
Unallocated
Financing Receivable, Impaired [Line Items]
Loans and leases receivable, allowance			1,045,000		1,588,000
Substandard
Financing Receivable, Impaired [Line Items]
Loans and leases receivable, before fees, gross, amount			$ 34,592,000		33,316,000
Originated Loans
Financing Receivable, Impaired [Line Items]
Percentage of real estate related loans			76.90%
Loans and leases receivable, before fees, gross, amount			$ 1,855,290,000		1,819,334,000
Loans and leases receivable, gross			1,855,290,000		1,819,334,000
Loans and leases receivable, allowance			25,357,000		25,290,000
Originated Loans | Substandard
Financing Receivable, Impaired [Line Items]
Loans and leases receivable, before fees, gross, amount			25,936,000		25,338,000
Acquired and Acquired Covered Loans
Financing Receivable, Impaired [Line Items]
Loans and leases receivable, life of loans		10 years
Loans and leases receivable, gross			$ 260,300,000
Risk weighting for covered assets under loss sharing agreement, percentage			20.00%
Risk weighting for covered assets not under loss sharing agreement, percentage			100.00%
Acquired and Acquired Covered Loans | Single Family
Financing Receivable, Impaired [Line Items]
Loss sharing agreement, term			10 years
Acquired and Acquired Covered Loans | Non-residential
Financing Receivable, Impaired [Line Items]
Loss sharing agreement, term			5 years
Loss sharing agreement, recovery period			3 years
Acquired and Acquired Covered Loans | Prime Pacific Financial Services
Financing Receivable, Impaired [Line Items]
Assets, fair value adjustment	$ 2,500,000		$ 1,600,000
Acquired and Acquired Covered Loans | Home Federal Bank
Financing Receivable, Impaired [Line Items]
Assets, fair value adjustment		$ 6,000,000	1,500,000
Acquired Covered Loans
Financing Receivable, Impaired [Line Items]
Loans and leases receivable, gross			18,200,000
Loan receivables
Financing Receivable, Impaired [Line Items]
Loan loss (provision)/ recovery			0	0
Loans and leases receivable, allowance			25,357,000	24,430,000	25,290,000	$ 24,415,000
Loan receivables | Unallocated
Financing Receivable, Impaired [Line Items]
Loan loss (provision)/ recovery			(543,000)	1,162,000
Loans and leases receivable, allowance			$ 1,045,000	$ 3,638,000	$ 1,588,000	$ 2,476,000