Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 18,329,017	$ 19,908,432
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	140	17,158
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,212,068	1,681,985
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	16,117,089	18,243,605
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	5,459,758	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	416,606	398,959
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	5,043,152	5,051,587
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	7,589,404	7,316,824
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	140	8,050
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,282,910	948,638
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	6,306,634	6,376,236
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	5,237,185	7,098,635
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	9,108
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	512,552	334,388
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	4,724,633	6,773,355
Other Investments [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	42,670	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	$ 42,670	$ 42,427