Stockholders' Equity - Accumulated other comprehensive income activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 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	727,757	808,897
Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	3,234	331
Other comprehensive income (loss) before reclassification	514	3,460
Amount reclassified from accumulated other comprehensive income	(851)	(557)
Net current period other comprehensive income (loss)	(337)	2,903
Balance at the end of the period	2,897	3,234
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	823	3,496
Amount reclassified from accumulated other comprehensive income	(648)	(178)
Net current period other comprehensive income (loss)	175	3,318
Balance at the end of the period	2,696	2,521
Postretirement benefits unrealized gains (losses)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	713	1,128
Other comprehensive income (loss) before reclassification	(309)	(36)
Amount reclassified from accumulated other comprehensive income	(203)	(379)
Net current period other comprehensive income (loss)	(512)	(415)
Balance at the end of the period	$ 201	$ 713