Basis of presentation and summary of significant accounting policies (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accounting Policies [Line Items]
Transfer to other real estate		$ 400,000	$ 1,600,000	$ 200,000
Transfer from fixed assets		900,000	400,000	5,900,000
Interest paid		2,100,000	1,900,000	2,200,000
Income taxes paid		700,000	1,100,000	0
Trading securities		0	0	0
Federal Home Loan Bank (FHLB) stock		$ 3,268,000	3,000,000
Threshold period past due for financing receivables		90 days
Loans and leases receivable, allowance		$ 25,290,000	24,415,000
MSRs		2,348,000	2,186,000	2,248,000
Bank owned life insurance		56,957,000	54,450,000
Other real estate owned (OREO), net		1,677,000	3,274,000	3,309,000	$ 3,144,000
Advertising expense		600,000	600,000	900,000
Deferred tax assets, valuation allowance		59,000	74,000
Swap fee income		2,483,000	2,533,000	1,847,000
Interest rate swap derivatives		5,239,000
Derivative notional amount		257,000,000	169,700,000
Assets held-in-trust		54,300,000	57,000,000
Finite-Lived Intangible Assets [Line Items]
Core deposit intangible, net		12,317,000	6,863,000
Interest Rate Swap
Accounting Policies [Line Items]
Interest rate swap derivatives	[1]	5,200,000	8,600,000
Interest rate swap derivatives		5,200,000	8,600,000
Separate Account BOLI
Accounting Policies [Line Items]
Bank owned life insurance		28,900,000	28,600,000
General Account BOLI
Accounting Policies [Line Items]
Bank owned life insurance		$ 28,000,000	25,900,000
One-to-Four Family Loans
Accounting Policies [Line Items]
Threshold period for one-to-four family loans to be deemed current		90 days
Unallocated
Accounting Policies [Line Items]
Loans and leases receivable, allowance		$ 1,588,000	2,476,000
Loans Receivable
Accounting Policies [Line Items]
Loans and leases receivable, allowance		25,290,000	24,415,000	22,053,000	20,857,000
Loans Receivable | Unallocated
Accounting Policies [Line Items]
Loans and leases receivable, allowance		1,588,000	2,476,000	5,294,000	714,000
Decrease in loans and leases receivable allowance		900,000
Unfunded Lending Commitments
Accounting Policies [Line Items]
Loans and leases receivable, allowance		440,000	440,000	440,000	440,000
Unfunded Lending Commitments | Unallocated
Accounting Policies [Line Items]
Loans and leases receivable, allowance		0	0	0	$ 0
Core Deposits
Finite-Lived Intangible Assets [Line Items]
Core deposit intangible, net		$ 12,317,000	$ 6,863,000	$ 7,683,000

[1] Included in other assets on the consolidated balance sheet