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		$ 121,167	$ 165,175	$ 121,622
Other comprehensive income (loss), net of tax
Net change in fair value of securities available for sale		8,857	10,345	(4,501)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax		16,797	10,331	4,829
Net change in other comprehensive income for defined benefit postretirement plans	[1]	(3,128)	(1,334)	1,558
Total other comprehensive income		22,526	19,342	1,886
Total comprehensive income		$ 143,693	$ 184,517	$ 123,508

[1] The amortization of prior service costs is included in the computation of net periodic pension costs as disclosed in Note 15 - Employee Benefit Plans.