Stockholders' Equity - Other comprehensive income (loss) activity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	$ 872,884
Balance at the end of the period	888,968
Accumulated Other Comprehensive Income(Loss)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	524	$ (6,757)
Amount reclassified to retained earnings for recently adopted ASUs	(848)
Other comprehensive income (loss) before reclassification	(1,131)	3,747
Amount reclassified from accumulated other comprehensive loss (income)	(23)	(177)
Net current period other comprehensive income (loss)	(2,002)	3,570
Balance at the end of the period	(1,478)	(3,187)
Unrealized (gains) losses on investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	145	(3,972)
Amount reclassified to retained earnings for recently adopted ASUs	(955)
Other comprehensive income (loss) before reclassification	(1,131)	1,423
Amount reclassified from accumulated other comprehensive loss (income)		(92)
Net current period other comprehensive income (loss)	(2,086)	1,331
Balance at the end of the period	(1,941)	(2,641)
Change in valuation allowance for unrealized gains (losses) on investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period		(3,388)
Other comprehensive income (loss) before reclassification		2,324
Amount reclassified from accumulated other comprehensive loss (income)		(56)
Net current period other comprehensive income (loss)		2,268
Balance at the end of the period		(1,120)
Postretirement benefits unrealized gains (losses)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	379	603
Amount reclassified to retained earnings for recently adopted ASUs	107
Amount reclassified from accumulated other comprehensive loss (income)	(23)	(29)
Net current period other comprehensive income (loss)	84	(29)
Balance at the end of the period	$ 463	$ 574