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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	Mar. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	$ 1,820,725	$ 1,945,833
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	63,577	107,063
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	1,884,302	2,052,896
Securities, held-to-maturity, allowance for credit loss	(158)	(160)
Debt Securities, Held-to-Maturity Net Carrying Amount	1,820,567	1,945,673
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	528,472	528,784
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	13,458	41,742
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	541,930	570,526
Securities, held-to-maturity, allowance for credit loss	0	0
Debt Securities, Held-to-Maturity Net Carrying Amount	528,472	528,784
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,290,753	1,415,549
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	50,119	65,321
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	1,340,872	1,480,870
Securities, held-to-maturity, allowance for credit loss	(158)	(160)
Debt Securities, Held-to-Maturity Net Carrying Amount	1,290,595	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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