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Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
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Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | $ 4,478,534 | $ 513,733
Unrealized losses, Less than 12 months | (49,992) | (3,190)
Estimated fair value, 12 months or longer | 53,668 | 102,106
Unrealized losses, 12 months or longer | (3,247) | (596)
Estimated fair value, Total | 4,532,202 | 615,839
Unrealized losses, Total | (53,239) | (3,786)
U.S. Treasury bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 52,927 | 7,744
Unrealized losses, Less than 12 months | (916) | (104)
Estimated fair value, 12 months or longer | 6,518 | 0
Unrealized losses, 12 months or longer | (291) | 0
Estimated fair value, Total | 59,445 | 7,744
Unrealized losses, Total | (1,207) | (104)
Municipal bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 570,038 | 74,045
Unrealized losses, Less than 12 months | (10,724) | (570)
Estimated fair value, 12 months or longer | 6,711 | 0
Unrealized losses, 12 months or longer | (282) | 0
Estimated fair value, Total | 576,749 | 74,045
Unrealized losses, Total | (11,006) | (570)
Foreign government bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 125,918 | 67,094
Unrealized losses, Less than 12 months | (1,094) | (516)
Estimated fair value, 12 months or longer | 29,032 | 0
Unrealized losses, 12 months or longer | (2,235) | 0
Estimated fair value, Total | 154,950 | 67,094
Unrealized losses, Total | (3,329) | (516)
Governmental agency bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 72,325 | 15,353
Unrealized losses, Less than 12 months | (469) | (195)
Estimated fair value, 12 months or longer | 3,222 | 0
Unrealized losses, 12 months or longer | (93) | 0
Estimated fair value, Total | 75,547 | 15,353
Unrealized losses, Total | (562) | (195)
Governmental agency mortgage-backed securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 3,070,056 | 287,947
Unrealized losses, Less than 12 months | (31,928) | (1,089)
Estimated fair value, 12 months or longer | 103 | 100,473
Unrealized losses, 12 months or longer | (1) | (579)
Estimated fair value, Total | 3,070,159 | 388,420
Unrealized losses, Total | (31,929) | (1,668)
U.S. corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 425,854 | 42,508
Unrealized losses, Less than 12 months | (3,388) | (484)
Estimated fair value, 12 months or longer | 6,848 | 1,357
Unrealized losses, 12 months or longer | (331) | (13)
Estimated fair value, Total | 432,702 | 43,865
Unrealized losses, Total | (3,719) | (497)
Foreign corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 161,416 | 19,042
Unrealized losses, Less than 12 months | (1,473) | (232)
Estimated fair value, 12 months or longer | 1,234 | 276
Unrealized losses, 12 months or longer | (14) | (4)
Estimated fair value, Total | 162,650 | 19,318
Unrealized losses, Total | $ (1,487) | $ (236)

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922890-210455 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27337-111563 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456 + Details
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