Securities (Securities Available for Sale) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 9,727,401	$ 10,877,769
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	710,168	405,399
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	4	13,577
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Available for Sale, Estimated Fair Value	10,437,565	11,269,591
US Treasury [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,084,542	1,941,283
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	35,091	18,934
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	12,084
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Available for Sale, Estimated Fair Value	1,119,633	1,948,133
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,916,581	2,176,275
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	71,102	32,608
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	4	1,289
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Available for Sale, Estimated Fair Value	1,987,679	2,207,594
States and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	6,683,927	6,717,344
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	603,975	353,857
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	204
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Available for Sale, Estimated Fair Value	7,287,902	7,070,997
Other [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	42,351	42,867
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Available for Sale, Estimated Fair Value	$ 42,351	$ 42,867

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455

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