Stockholders' Equity - Accumulated other comprehensive income (loss) activity (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	$ 868,604	$ 888,968	$ 883,450	$ 872,884
Balance at the end of the period	856,656	885,718	856,656	885,718
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	1,895	(1,478)	331	524
Amount reclassified to retained earnings for ASUs adopted in 2018				(848)
Other comprehensive income (loss) before reclassification	1,281	169	3,034	(962)
Amount reclassified from accumulated other comprehensive income (loss)	(95)	(23)	(284)	(46)
Net current period other comprehensive income (loss)	1,186	146	2,750	(1,856)
Balance at the end of the period	3,081	(1,332)	3,081	(1,332)
Unrealized gains (losses) on AFS investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	861	(1,941)	(797)	145
Amount reclassified to retained earnings for ASUs adopted in 2018				(955)
Other comprehensive income (loss) before reclassification	1,281	169	3,034	(962)
Amount reclassified from accumulated other comprehensive income (loss)			(95)
Net current period other comprehensive income (loss)	1,281	169	2,939	(1,917)
Balance at the end of the period	2,142	(1,772)	2,142	(1,772)
Postretirement benefits unrealized gains (losses)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	1,034	463	1,128	379
Amount reclassified to retained earnings for ASUs adopted in 2018				107
Amount reclassified from accumulated other comprehensive income (loss)	(95)	(23)	(189)	(46)
Net current period other comprehensive income (loss)	(95)	(23)	(189)	61
Balance at the end of the period	$ 939	$ 440	$ 939	$ 440