Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 11,230,837	$ 9,727,401
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	530,402	710,168
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	30,017	4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	11,731,222	10,437,565
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,670,692	1,084,542
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	20,609	35,091
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6,080	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	1,685,221	1,119,633
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	2,467,915	1,916,581
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	37,910	71,102
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	14,671	4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	2,491,154	1,987,679
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	7,049,872	6,683,927
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	471,883	603,975
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	9,266	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	7,512,489	7,287,902
Other Investments [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	42,358	42,351
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, Held-to-maturity, Allowance for Credit Loss	0	0
Securities available for sale, at estimated fair value	$ 42,358	$ 42,351

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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