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Investments - Maturity Table (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,116,279	$ 1,986,957
Estimated Fair Value	2,115,885	$ 1,996,634
2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	293,759
Estimated Fair Value	294,232
2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,018,268
Estimated Fair Value	1,019,960
2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	148,822
Estimated Fair Value	149,166
2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	100,000
Estimated Fair Value	99,925
2038
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	76
Estimated Fair Value	88
2039
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,601
Estimated Fair Value	1,773
2042
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,201
Estimated Fair Value	4,206
2043
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,415
Estimated Fair Value	7,711
2044
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,133
Estimated Fair Value	10,610
2045
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,870
Estimated Fair Value	9,179
2046
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	14,414
Estimated Fair Value	14,797
2047
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	18,646
Estimated Fair Value	18,992
2048
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,362
Estimated Fair Value	4,545
2049
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	36,368
Estimated Fair Value	37,683
2050
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	174,919
Estimated Fair Value	170,740
2051
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	75,449
Estimated Fair Value	73,143
2053
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	134,414
Estimated Fair Value	134,289
2054
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	64,562
Estimated Fair Value	$ 64,846

X

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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

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