Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 18,860,732	$ 19,908,432
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6,203	17,158
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,617,678	1,681,985
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	17,249,257	18,243,605
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	5,456,656	5,450,546
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	383,768	398,959
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	5,072,888	5,051,587
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	7,725,403	7,316,824
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5,105	8,050
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	937,733	948,638
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	6,792,775	6,376,236
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	5,636,095	7,098,635
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,098	9,108
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	296,177	334,388
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	5,341,016	6,773,355
Other Investments [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	42,578	42,427
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	$ 42,578	$ 42,427