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Investments - Maturity Table (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,506,087	$ 1,986,957
Estimated Fair Value	2,505,372	$ 1,996,634
2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	16,029
Estimated Fair Value	16,045
2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	850,772
Estimated Fair Value	851,858
2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	162,641
Estimated Fair Value	162,906
2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	448,293
Estimated Fair Value	447,649
2026
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	447,886
Estimated Fair Value	447,991
2038
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	74
Estimated Fair Value	80
2039
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,079
Estimated Fair Value	1,187
2042
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,421
Estimated Fair Value	3,443
2043
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,121
Estimated Fair Value	6,394
2044
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,826
Estimated Fair Value	8,164
2045
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,143
Estimated Fair Value	7,418
2046
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,898
Estimated Fair Value	11,170
2047
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,598
Estimated Fair Value	13,935
2048
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,966
Estimated Fair Value	3,070
2049
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	24,506
Estimated Fair Value	25,449
2050
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	148,550
Estimated Fair Value	145,645
2051
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	149,307
Estimated Fair Value	146,869
2053
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	148,730
Estimated Fair Value	149,218
2054
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	56,247
Estimated Fair Value	$ 56,881

X

- 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=123581744&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
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+ References

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