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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, 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 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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