Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	$ 1,951,544	$ 2,030,005
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	107,312	28,931
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	1	10,261
Debt Securities, Held-to-maturity, Fair Value	2,058,855	2,048,675
Securities, held-to-maturity, allowance for credit loss	(160)	0
Debt Securities, Held-to-Maturity Net Carrying Amount	1,951,384	2,030,005
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	529,290	530,861
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	45,504	22
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	9,365
Debt Securities, Held-to-maturity, Fair Value	574,794	521,518
Securities, held-to-maturity, allowance for credit loss	0	0
Debt Securities, Held-to-Maturity Net Carrying Amount	529,290	530,861
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,420,754	1,497,644
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	61,808	28,909
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	896
Debt Securities, Held-to-maturity, Fair Value	1,482,562	1,525,657
Securities, held-to-maturity, allowance for credit loss	(160)	0
Debt Securities, Held-to-Maturity Net Carrying Amount	1,420,594	1,497,644
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	1	0
Debt Securities, Held-to-maturity, Fair Value	1,499	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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- Definition

Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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