Loans and reserve for credit losses (Narrative) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Percentage of real estate related loans 72.80%    
Loans participated to third-parties $ 34,500,000 $ 11,300,000  
Loans receivable 1,492,540,000 996,232,000  
Total loans, percent 100.00%    
Loan loss provision 0 1,000,000 1,100,000
Total accruing loans which are contractually past due 90 days or more 54,000 1,083,000  
Impaired loans, total recorded balance 33,722,000 40,467,000  
Loans and leases receivable, allowance 22,053,000 20,857,000  
Troubled Debt Restructuring      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Impaired loans, total recorded balance 22,800,000 33,200,000  
Loans and leases receivable, allowance 100,000 800,000  
Loans and leases receivable, impaired, commitment to lend 0 0  
Originated Loans, Receivable      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Percentage of real estate related loans 68.10%    
Loans receivable 1,182,690,000 [1] 996,232,000 [1]  
Total loans, percent 100.00% [1] 100.00% [1]  
Loans and leases receivable, allowance 22,053,000 [1] 20,857,000 [1]  
Acquired and Acquired Covered Loans, Receivable      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Increase (decrease) in acquired and acquired covered loans (6,000,000)    
Loans And Leases Receivable, Life Of Loans 10 years    
Loans receivable 309,900,000    
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%    
Percentage of acquired loans covered 13.00%    
Acquired Covered Loans, Receivable      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans receivable 40,191,000 [2]    
Total loans, percent 100.00% [2]    
Loans receivable net reported amount covered 40,200,000    
Substandard      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans receivable 38,635,000 41,194,000  
Substandard | Originated Loans, Receivable      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans receivable 29,582,000 [1]    
Substandard | Acquired and Acquired Covered Loans, Receivable      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans receivable 9,100,000    
Total loans, percent 2.90%    
Substandard | Acquired Covered Loans, Receivable      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans receivable 2,912,000 [2]    
Loans receivable net reported amount covered 2,900,000    
Loans receivable covered percent 32.20%    
Special Mention and Substandard [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans And Leases Receivable, Gross, Increase (Decrease) In Loans 13,100,000    
Special Mention and Substandard [Member] | Originated Loans, Receivable      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans And Leases Receivable, Gross, Increase (Decrease) In Loans (18,200,000)    
Unallocated      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans and leases receivable, allowance $ 5,294,000 $ 714,000  
Non-residential | Acquired and Acquired Covered Loans, Receivable      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loss Sharing Agreement, Term 5 years    
Loss Sharing Agreement, Recovery Period 3 years    
Single Family | Acquired and Acquired Covered Loans, Receivable      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loss Sharing Agreement, Term 10 years    
[1] Originated loans are loans organically made through the Company's normal and customary origination
[2] Acquired covered loans acquired in the acquisition of Home that are covered under FDIC loss share