Accumulated Other Comprehensive Loss - Reclassification from Each Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Reclassification Out Of Accumulated Other Comprehensive Loss
Unrealized losses on AFS securities before tax	$ 375		$ (41)		$ 476	$ (81)
Tax effect	(42,881)		(33,466)		(82,202)	(62,595)
Net income	120,512	$ 115,710	95,601	$ 87,268	236,222	182,869
Interest expense	(32,741)		(65,966)		(71,872)	(164,420)
Amortization of actuarial net loss	17,928		18,522		36,321	35,061
Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Out Of Accumulated Other Comprehensive Loss
Net income	(888)		105		(1,863)	(532)
Interest expense	(749)		438		(1,664)	(177)
Amounts Reclassified from Accumulated Other Comprehensive Loss | Unrealized losses on AFS securities before tax
Reclassification Out Of Accumulated Other Comprehensive Loss
Unrealized losses on AFS securities before tax	(103)		(41)		(162)	(81)
Tax effect	27		10		42	23
Net income	(76)		(31)		(120)	(58)
Amounts Reclassified from Accumulated Other Comprehensive Loss | Unrealized (losses) gains on derivatives (cash flow hedges) before tax
Reclassification Out Of Accumulated Other Comprehensive Loss
Tax effect	215		(130)		479	47
Net income	(534)		308		(1,185)	(130)
Interest expense	(749)		438		(1,664)	(177)
Amounts Reclassified from Accumulated Other Comprehensive Loss | Defined benefit pension plan
Reclassification Out Of Accumulated Other Comprehensive Loss
Tax effect	109		57		217	121
Net income	(278)		(172)		(558)	(344)
Amortization of actuarial net loss	$ (387)		$ (229)		$ (775)	$ (465)