Loans and reserve for credit losses (Narrative) (Details)			3 Months Ended		9 Months Ended
	Aug. 01, 2016 USD ($)	May 16, 2014 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) payment	Sep. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Financing Receivable, Impaired [Line Items]
Percentage of real estate related loans			78.20%		78.20%
Loans participated to third-parties			$ 50,700,000		$ 50,700,000			$ 44,200,000
Loans and leases receivable, before fees, gross, amount			$ 2,061,508,000		$ 2,061,508,000			1,687,929,000
Percentage of loans acquired, substandard			3.30%		3.30%
Loans and leases receivable, gross			$ 2,061,508,000		$ 2,061,508,000			1,687,929,000
Percentage of acquired loans covered			7.50%		7.50%
Loan loss recovery			$ 0	$ 0	$ 0	$ (2,000,000)
Loans and leases receivable, allowance			25,238,000		25,238,000			24,415,000
Loans and leases receivable, additional allowance			0		0
Total accruing loans which are contractually past due 90 days or more			59,000		59,000			68,000
Impaired loans, total recorded balance			11,800,000		11,800,000			6,942,000
Interest income recognized for cash payments received on impaired loans			500,000		1,100,000
TDR outstanding recorded investment			22,000	0	22,000	0
Troubled Debt Restructuring
Financing Receivable, Impaired [Line Items]
Loan loss recovery					0	0
Loans and leases receivable, allowance			0		0			0
Impaired loans, total recorded balance			600,000		$ 600,000			800,000
Number of payments before upgrade to accrual status review | payment					6
Loans and leases receivable, impaired, commitment to lend			0	0	$ 0	0
TDR outstanding recorded investment			0	0	0	0
Unallocated
Financing Receivable, Impaired [Line Items]
Loans and leases receivable, allowance			1,838,000		1,838,000			2,476,000
Substandard
Financing Receivable, Impaired [Line Items]
Loans and leases receivable, before fees, gross, amount			$ 41,208,000		$ 41,208,000			39,499,000
Originated Loans
Financing Receivable, Impaired [Line Items]
Percentage of real estate related loans			76.10%		76.10%
Loans and leases receivable, before fees, gross, amount			$ 1,762,724,000		$ 1,762,724,000			1,452,978,000
Loans and leases receivable, gross			1,762,724,000		1,762,724,000			1,452,978,000
Loans and leases receivable, allowance			25,238,000		25,238,000			24,415,000
Originated Loans | Substandard
Financing Receivable, Impaired [Line Items]
Loans and leases receivable, before fees, gross, amount			31,410,000		31,410,000			25,802,000
Acquired and Acquired Covered Loans
Financing Receivable, Impaired [Line Items]
Loans and leases receivable, life of loans		10 years
Loans and leases receivable, gross			298,800,000		$ 298,800,000
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	$ 2,300,000				$ 2,300,000
Acquired and Acquired Covered Loans | Home Federal Bank
Financing Receivable, Impaired [Line Items]
Assets, fair value adjustment		$ 6,000,000			1,900,000
Acquired Covered Loans
Financing Receivable, Impaired [Line Items]
Loans and leases receivable, gross			22,300,000		22,300,000
Loan receivables
Financing Receivable, Impaired [Line Items]
Loan loss recovery			0	0	0	(2,000,000)
Loans and leases receivable, allowance			25,238,000	26,623,000	25,238,000	26,623,000	$ 24,666,000	24,415,000	$ 23,501,000	$ 22,053,000
Loan receivables | Unallocated
Financing Receivable, Impaired [Line Items]
Loan loss recovery			(1,544,000)	3,091,000	(638,000)	91,000
Loans and leases receivable, allowance			$ 1,838,000	$ 5,385,000	$ 1,838,000	$ 5,385,000	$ 3,382,000	$ 2,476,000	$ 2,294,000	$ 5,294,000