Investment Securities Summary of Amortized Cost and Fair Values of Corporate Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 3,009,064
Estimated Fair Value	3,097,375	$ 3,062,143
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	354,593	348,391
Estimated Fair Value	$ 371,381	$ 367,145

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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