Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 83,651	$ 167,596	$ 146,920
Securities available for sale:
Unrealized holding gains (losses) on securities	20,717	18,625	(8,315)
Reclassification adjustment for (gains) losses realized in net income	(34)	1,872	12
Total securities available for sale	20,683	20,497	(8,303)
Derivative instruments:
Unrealized holding gains (losses) on derivative instruments	688	(2,217)	365
Reclassification adjustment for losses realized in net income related to swap termination	1,521	0	0
Total derivative instruments	2,209	(2,217)	365
Defined benefit pension and post-retirement benefit plans:
Net gain arising during the period	797	68	308
Reclassification adjustment for settlement loss related to the VERP realized in net income	422	0	0
New prior service cost	(362)	0	0
Amortization of net actuarial loss recognized in net periodic pension cost	193	312	245
Amortization of prior service cost	362	0	0
Total defined benefit pension and post-retirement benefit plans	1,412	380	553
Other comprehensive income (loss), net of tax	24,304	18,660	(7,385)
Comprehensive income	$ 107,955	$ 186,256	$ 139,535