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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 | Jun. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Schedule of Investments [Line Items] |  |
Held to Maturity, Amortized Cost | $ 1,813,563 | $ 1,945,833
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain | 82,673 | 107,063
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss | 1 | 0
Debt Securities, Held-to-maturity, Fair Value | 1,896,235 | 2,052,896
Securities, held-to-maturity, allowance for credit loss | (158) | (160)
Debt Securities, Held-to-Maturity Net Carrying Amount | 1,813,405 | 1,945,673
Residential Mortgage Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Held to Maturity, Amortized Cost | 527,958 | 528,784
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain | 30,129 | 41,742
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss | 1 | 0
Debt Securities, Held-to-maturity, Fair Value | 558,086 | 570,526
Securities, held-to-maturity, allowance for credit loss | 0 | 0
Debt Securities, Held-to-Maturity Net Carrying Amount | 527,958 | 528,784
US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Held to Maturity, Amortized Cost | 1,284,105 | 1,415,549
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain | 52,544 | 65,321
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss | 0 | 0
Debt Securities, Held-to-maturity, Fair Value | 1,336,649 | 1,480,870
Securities, held-to-maturity, allowance for credit loss | (158) | (160)
Debt Securities, Held-to-Maturity Net Carrying Amount | 1,283,947 | 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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- 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 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aaa) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447 + Details
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