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Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Available-for-sale |  |
Amortized cost | $ 5,145,866 | $ 2,521,374
Gross unrealized gains | 192,740 | 55,965
Gross unrealized losses | (792) | (2,087)
Total | 5,337,814 | 2,575,252
Held-to-maturity |  |
Amortized cost | 189,836 | 224,611
Gross unrealized gains | 13,380 | 9,785
Gross unrealized losses | 0 | 0
Fair value | 203,216 | 234,396
Debt Securities, Available-for-sale and Held-to-maturity [Abstract] |  |
Amortized cost | 5,335,702 | 2,745,985
Gross unrealized gains | 206,120 | 65,750
Gross unrealized losses | (792) | (2,087)
Fair value | 5,541,030 | 2,809,648
U.S. government and federal agency |  |
Available-for-sale |  |
Amortized cost | 38,568 | 20,061
Gross unrealized gains | 287 | 48
Gross unrealized losses | (267) | (65)
Total | 38,588 | 20,044
U.S. government sponsored enterprises |  |
Available-for-sale |  |
Amortized cost | 9,747 | 42,724
Gross unrealized gains | 34 | 953
Gross unrealized losses | 0 | 0
Total | 9,781 | 43,677
State and local governments |  |
Available-for-sale |  |
Amortized cost | 1,321,763 | 679,784
Gross unrealized gains | 94,974 | 22,694
Gross unrealized losses | (54) | (80)
Total | 1,416,683 | 702,398
Held-to-maturity |  |
Amortized cost | 189,836 | 224,611
Gross unrealized gains | 13,380 | 9,785
Gross unrealized losses | 0 | 0
Fair value | 203,216 | 234,396
Corporate bonds |  |
Available-for-sale |  |
Amortized cost | 336,867 | 155,665
Gross unrealized gains | 12,239 | 1,938
Gross unrealized losses | (8) | (1)
Total | 349,098 | 157,602
Residential mortgage-backed securities |  |
Available-for-sale |  |
Amortized cost | 2,261,463 | 731,766
Gross unrealized gains | 27,631 | 7,507
Gross unrealized losses | (4) | (549)
Total | 2,289,090 | 738,724
Commercial mortgage-backed securities |  |
Available-for-sale |  |
Amortized cost | 1,177,458 | 891,374
Gross unrealized gains | 57,575 | 22,825
Gross unrealized losses | (459) | (1,392)
Total | $ 1,234,574 | $ 912,807

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- Definition This item represents the cost of available-for-sale and held-to-maturity debt securities, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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- Definition Fair value of investments in debt securities classified as available-for sale and held-to-maturity. + References No definition available. + Details
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- Definition Amount of accumulated unrealized gain before deducting unrealized loss on investments in available-for-sale and held-to-maturity securities impacting investments. + References No definition available. + Details
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- Definition Amount of accumulated unrealized loss before deducting unrealized gain on investments in available-for-sale and held-to-maturity securities impacting investments. + References No definition available. + Details
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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454 + Details
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- Definition 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 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -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 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 + Details
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- Definition Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=SL120269820-111563 + Details
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