Basis of presentation and summary of significant accounting policies (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jun. 30, 2013
Dec. 31, 2011
Accounting Policies [Line Items]          
Transfer to other real estate $ 200,000 $ 2,100,000 $ 1,300,000    
Interest paid 8,100,000 3,000,000 5,100,000    
Income taxes paid 0 100,000 100,000    
Trading securities 0 0 0    
Sale of excess capital stock 8,812 3,722      
Proceeds from sale of federal reserve stock 900,000 400,000      
Threshold period past due for financing receivables 90 days        
Loans and leases receivable, allowance 22,053,000 20,857,000      
MSRs 2,248,000 2,232,000 1,308,000    
Bank owned life insurance 53,449,000 36,567,000      
Other real estate owned (OREO), net 3,309,000 3,144,000 6,552,000   21,270,000
Advertising expense 900,000 1,000,000 1,000,000    
Deferred tax assets, valuation allowance (78,000) (74,000) (41,700,000)    
Previously written off section 382 deferred tax benefits       8,500,000  
Swap fee income 1,847,000 430,000 0    
Interest rate swap derivatives 4,828,000        
Derivative notional amount 82,900,000 16,000,000      
Assets held-in-trust 58,500,000 59,600,000      
Preferred stock, shares authorized 5,000,000 5,000,000      
Preferred stock, shares issues 0 0      
Preferred stock, shares outstanding 0 0      
Finite-Lived Intangible Assets [Line Items]          
Core deposit intangible, net 7,683,000 529,000      
Core Deposits          
Finite-Lived Intangible Assets [Line Items]          
Core deposit intangible, net 7,683,000 529,000 0    
One-to-Four Family Loans          
Accounting Policies [Line Items]          
Threshold period for one-to-four family loans to be deemed current 90 days        
Separate Account BOLI          
Accounting Policies [Line Items]          
Bank owned life insurance 28,300,000 27,900,000      
General Account BOLI          
Accounting Policies [Line Items]          
Bank owned life insurance 25,100,000 8,700,000      
Unfunded Lending Commitments          
Accounting Policies [Line Items]          
Loans and leases receivable, allowance 440,000 440,000 440,000   1,550,000
Interest Rate Swap          
Accounting Policies [Line Items]          
Interest rate swap derivatives 4,800,000 [1]        
Interest rate swap derivatives $ 4,800,000        
[1] Included in other assets on the consolidated balance sheet