Stockholders' Equity - Accumulated other comprehensive income (loss) activity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	$ 883,450	$ 872,884
Balance at the end of the period	868,604	888,968
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	331	524
Amount reclassified to retained earnings for ASUs adopted in 2018		(848)
Other comprehensive income (loss) before reclassification	1,753	(1,131)
Amount reclassified from accumulated other comprehensive income (loss)	(189)	(23)
Net current period other comprehensive income (loss)	1,564	(2,002)
Balance at the end of the period	1,895	(1,478)
Unrealized gains (losses) on AFS investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(797)	145
Amount reclassified to retained earnings for ASUs adopted in 2018		(955)
Other comprehensive income (loss) before reclassification	1,753	(1,131)
Amount reclassified from accumulated other comprehensive income (loss)	(95)
Net current period other comprehensive income (loss)	1,658	(2,086)
Balance at the end of the period	861	(1,941)
Postretirement benefits unrealized gains (losses)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	1,128	379
Amount reclassified to retained earnings for ASUs adopted in 2018		107
Amount reclassified from accumulated other comprehensive income (loss)	(94)	(23)
Net current period other comprehensive income (loss)	(94)	84
Balance at the end of the period	$ 1,034	$ 463