Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Held to Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	$ 1,984,349	$ 2,030,005
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	73,530	28,931
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	10,261
Debt Securities, Held-to-maturity, Fair Value	2,057,879	2,048,675
Debt Securities, Held-to-maturity, Allowance for Credit Loss	(200)	0
Debt Securities, Held-to-Maturity Net Carrying Amount	1,984,149	2,030,005
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	530,396	530,861
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	37,234	22
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	9,365
Debt Securities, Held-to-maturity, Fair Value	567,630	521,518
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Debt Securities, Held-to-Maturity Net Carrying Amount	530,396	530,861
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,452,453	1,497,644
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	36,296	28,909
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	896
Debt Securities, Held-to-maturity, Fair Value	1,488,749	1,525,657
Debt Securities, Held-to-maturity, Allowance for Credit Loss	(200)	0
Debt Securities, Held-to-Maturity Net Carrying Amount	1,452,253	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	0	0
Debt Securities, Held-to-maturity, Fair Value	1,500	1,500
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Debt Securities, Held-to-Maturity Net Carrying Amount	$ 1,500	$ 1,500

Debt Securities, Held-to-Maturity Net Carrying Amount

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
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-Section 50
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Amount of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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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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- Definition

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

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- Definition

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

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