Stockholders' Equity - Accumulated other comprehensive income (loss) activity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	$ 856,656	$ 885,718	$ 883,450	$ 872,884
Balance at the end of the period	839,530	893,510	839,530	893,510
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	3,081	(1,332)	331	524
Amount reclassified to retained earnings for ASUs adopted in 2018				(848)
Other comprehensive income (loss) before reclassification	379	218	3,413	(744)
Amount reclassified from accumulated other comprehensive income (loss)	(177)	20	(461)	(26)
Net current period other comprehensive income (loss)	202	238	2,952	(1,618)
Balance at the end of the period	3,283	(1,094)	3,283	(1,094)
Unrealized gains (losses) on AFS investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	2,142	(1,772)	(797)	145
Amount reclassified to retained earnings for ASUs adopted in 2018				(955)
Other comprehensive income (loss) before reclassification	379	218	3,413	(744)
Amount reclassified from accumulated other comprehensive income (loss)	(83)	44	(178)	44
Net current period other comprehensive income (loss)	296	262	3,235	(1,655)
Balance at the end of the period	2,438	(1,510)	2,438	(1,510)
Postretirement benefits unrealized gains (losses)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	939	440	1,128	379
Amount reclassified to retained earnings for ASUs adopted in 2018				107
Amount reclassified from accumulated other comprehensive income (loss)	(94)	(24)	(283)	(70)
Net current period other comprehensive income (loss)	(94)	(24)	(283)	37
Balance at the end of the period	$ 845	$ 416	$ 845	$ 416