Stockholders' Equity - Other comprehensive income (loss) activity (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period			$ 872,884
Balance at the end of the period	$ 893,510		893,510
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(1,332)	$ (1,980)	524	$ (6,757)
Amount reclassified to retained earnings for recently adopted ASUs			(848)
Other comprehensive income (loss) before reclassification	218	2,768	(744)	7,856
Amount reclassified from accumulated other comprehensive loss	20	(728)	(26)	(1,039)
Net current period other comprehensive income (loss)	238	2,040	(1,618)	6,817
Balance at the end of the period	(1,094)	60	(1,094)	60
Unrealized gains (losses) on investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(1,772)	(1,986)	145	(3,972)
Amount reclassified to retained earnings for recently adopted ASUs			(955)
Other comprehensive income (loss) before reclassification	218	1,968	(744)	4,113
Amount reclassified from accumulated other comprehensive loss	44	(438)	44	(597)
Net current period other comprehensive income (loss)	262	1,530	(1,655)	3,516
Balance at the end of the period	(1,510)	(456)	(1,510)	(456)
Change in valuation allowance for unrealized gains (losses) on investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period		(540)		(3,388)
Other comprehensive income (loss) before reclassification		800		3,743
Amount reclassified from accumulated other comprehensive loss		(260)		(355)
Net current period other comprehensive income (loss)		540		3,388
Postretirement benefits unrealized gains (losses)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	440	546	379	603
Amount reclassified to retained earnings for recently adopted ASUs			107
Amount reclassified from accumulated other comprehensive loss	(24)	(30)	(70)	(87)
Net current period other comprehensive income (loss)	(24)	(30)	37	(87)
Balance at the end of the period	$ 416	$ 516	$ 416	$ 516

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of the cumulative effect on retained earnings net of related income tax effect.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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Amount after tax and reclassifications from recently adopted ASUS of other comprehensive income (loss) attributable to parent entity.

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