Income taxes (Narrative) (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended								12 Months Ended
	Jun. 30, 2013	Jan. 31, 2011	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Loss Carryforwards [Line Items]
Effective income tax rate											37.80%
Company's pretax income			$ 9,445	$ 7,725	$ 7,656	$ 8,266	$ 8,022	$ 4,397	$ (9,743)	$ 1,239	$ 33,092	$ 3,915	$ 699
Nondeductible merger related costs											0	974	0
Income tax expense (benefit)			3,878	$ 2,626	$ 2,861	$ 3,148	2,982	$ 1,965	$ (5,065)	$ 296	12,513	178	(50,146)
Reversal of valuation allowance	$ 41,600										(4)	0	(41,632)
Previously written off section 382 deferred tax benefits													8,500
Deferred tax assets, operating loss carryforwards			22,100				32,100				22,100	32,100
Deferred tax assets, operating loss carryforwards state and local			6,000				6,600				6,000	6,600
Deferred tax assets, charitable contribution carryforwards			0				800				0	800
Deferred tax assets, federal tax credit carryforwards			2,300				1,300				2,300	1,300
Deferred tax assets, state tax credit carryforwards			200				100				200	100
Charitable contribution carryforward amount			100								100
Valuation allowance for deferred tax assets			(74)				$ (78)				(74)	$ (78)	$ (100)	$ (41,700)
Proceeds from Issuance of Common Stock		$ 177,000
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards			63,000								63,000
Tax credit carryforward			2,300								2,300
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards			119,400								119,400
Tax credit carryforward			$ 200								$ 200