Stockholders' Equity - Accumulated other comprehensive income (loss) activity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	$ 808,897	$ 883,450
Balance at the end of the period	765,658	868,604
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	3,234	331
Other comprehensive income (loss) before reclassification	(2,228)	1,753
Amount reclassified from accumulated other comprehensive income (loss)	(251)	(189)
Net current period other comprehensive income (loss)	(2,479)	1,564
Balance at the end of the period	755	1,895
Unrealized gains (losses) on AFS investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	2,521	(797)
Other comprehensive income (loss) before reclassification	(2,228)	1,753
Amount reclassified from accumulated other comprehensive income (loss)	(202)	(95)
Net current period other comprehensive income (loss)	(2,430)	1,658
Balance at the end of the period	91	861
Postretirement benefits unrealized gains (losses)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	713	1,128
Amount reclassified from accumulated other comprehensive income (loss)	(49)	(94)
Net current period other comprehensive income (loss)	(49)	(94)
Balance at the end of the period	$ 664	$ 1,034