Stockholders' Equity - Other comprehensive income (loss) activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	$ 872,884	$ 844,002
Balance at the end of the period	883,450	872,884
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	524	(6,757)
Amount reclassified to retained earnings for ASUs adopted in 2018	(848)
Other comprehensive income (loss) before reclassification	376	7,676
Amount reclassified from accumulated other comprehensive loss	279	(395)
Net current period other comprehensive income (loss)	(193)	7,281
Balance at the end of the period	331	524
Unrealized gains (losses) on investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	145	(3,972)
Amount reclassified to retained earnings for ASUs adopted in 2018	(955)
Other comprehensive income (loss) before reclassification	(360)	4,039
Amount reclassified from accumulated other comprehensive loss	373	78
Net current period other comprehensive income (loss)	(942)	4,117
Balance at the end of the period	(797)	145
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		(3,388)
Other comprehensive income (loss) before reclassification		3,743
Amount reclassified from accumulated other comprehensive loss		(355)
Net current period other comprehensive income (loss)		3,388
Postretirement benefits unrealized gains (losses)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	379	603
Amount reclassified to retained earnings for ASUs adopted in 2018	107
Other comprehensive income (loss) before reclassification	736	(106)
Amount reclassified from accumulated other comprehensive loss	(94)	(118)
Net current period other comprehensive income (loss)	749	(224)
Balance at the end of the period	$ 1,128	$ 379