Accumulated Other Comprehensive Loss (Reclassifications out of AOCI) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	$ 60,253	$ 7,115	$ 351
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	(13,858)	(1,636)	$ (81)
Amounts reclassified from accumulated other comprehensive income, net of tax	(27,276)	18,292
Unrealized gains on securities available for sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	60,253	7,115
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	(13,858)	(1,636)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	46,395	5,479
Amounts reclassified from accumulated other comprehensive income, net of tax	(46,395)	(5,479)
Unrealized gains on securities available for sale transferred from (to) held to maturity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss) Reclassification Adjustment Of Unrealized Losses From AOCI For Securities Available For Sale Transferred To Held To Maturity Before Tax	495	(19,889)
Other Comprehensive Income (Loss) Reclassification Adjustment Of Unrealized Losses From AOCI For Securities Available For Sale Transferred To Held To Maturity Tax	(114)	4,574
Other Comprehensive Income (Loss) Reclassification Adjustment Of Unrealized Losses From AOCI For Securities Available For Sale Transferred To Held To Maturity Net Of Tax	381	(15,315)
Amounts reclassified from accumulated other comprehensive income, net of tax	(381)	15,315
Defined Benefit Pension Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax		(57)
Reclassification adjustment for losses included in income before income taxes		10,924
Total before taxes	25,324	10,981
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax	5,824	2,525
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, after Tax	19,500	8,456
Amounts reclassified from accumulated other comprehensive income, net of tax	$ 19,500	$ 8,456