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Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	$ 1,759,428	$ 1,945,833
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	66,172	107,063
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	32	0
Debt Securities, Held-to-maturity, Fair Value	1,825,568	2,052,896
Securities, held-to-maturity, allowance for credit loss	(158)	(160)
Debt Securities, Held-to-Maturity Net Carrying Amount	1,759,270	1,945,673
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	527,649	528,784
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	24,332	41,742
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	551,981	570,526
Securities, held-to-maturity, allowance for credit loss	0	0
Debt Securities, Held-to-Maturity Net Carrying Amount	527,649	528,784
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,230,279	1,415,549
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	41,840	65,321
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	32	0
Debt Securities, Held-to-maturity, Fair Value	1,272,087	1,480,870
Securities, held-to-maturity, allowance for credit loss	(158)	(160)
Debt Securities, Held-to-Maturity Net Carrying Amount	1,230,121	1,415,389
Other Investments [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,500	1,500
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	1,500	1,500
Securities, held-to-maturity, allowance for credit loss	0	0
Debt Securities, Held-to-Maturity Net Carrying Amount	$ 1,500	$ 1,500

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