Investments, Investments Temporarily Impaired (Details) $ in Thousands	Sep. 30, 2021 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments \| Investment	616
Estimated fair value, less than 12 months of temporary impairment	$ 647,954
Unrealized losses, less than 12 months of temporary impairment	$ (865)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments \| Investment	158
Estimated fair value, less than 12 months of temporary impairment	$ 334,828
Unrealized losses, less than 12 months of temporary impairment	$ (525)
Debt Securities Issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments \| Investment	13
Estimated fair value, less than 12 months of temporary impairment	$ 103,460
Unrealized losses, less than 12 months of temporary impairment	$ (127)
Debt Securities Issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments \| Investment	10
Estimated fair value, less than 12 months of temporary impairment	$ 75,009
Unrealized losses, less than 12 months of temporary impairment	$ (90)
Debt Securities Issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments \| Investment	432
Estimated fair value, less than 12 months of temporary impairment	$ 123,489
Unrealized losses, less than 12 months of temporary impairment	$ (95)
Other Municipal Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments \| Investment	3
Estimated fair value, less than 12 months of temporary impairment	$ 11,168
Unrealized losses, less than 12 months of temporary impairment	$ (28)

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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 less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on 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 less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

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