Stockholders' Equity - Accumulated other comprehensive income (loss) activity (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	$ 765,658	$ 868,604	$ 808,897	$ 883,450
Balance at the end of the period	766,703	856,656	766,703	856,656
Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	755	1,895	3,234	331
Other comprehensive income (loss) before reclassification	3,943	1,281	1,716	3,034
Amount reclassified from accumulated other comprehensive income (loss)	(211)	(95)	(463)	(284)
Net current period other comprehensive income (loss)	3,732	1,186	1,253	2,750
Balance at the end of the period	4,487	3,081	4,487	3,081
Unrealized gains (losses) on AFS investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	91	861	2,521	(797)
Other comprehensive income (loss) before reclassification	3,943	1,281	1,716	3,034
Amount reclassified from accumulated other comprehensive income (loss)	(160)		(363)	(95)
Net current period other comprehensive income (loss)	3,783	1,281	1,353	2,939
Balance at the end of the period	3,874	2,142	3,874	2,142
Postretirement benefits unrealized gains (losses)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	664	1,034	713	1,128
Amount reclassified from accumulated other comprehensive income (loss)	(51)	(95)	(100)	(189)
Net current period other comprehensive income (loss)	(51)	(95)	(100)	(189)
Balance at the end of the period	$ 613	$ 939	$ 613	$ 939