Investments, Investments Temporarily Impaired (Details) $ in Thousands	Mar. 31, 2021 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	414
Estimated fair value, less than 12 months of temporary impairment	$ 435,239
Unrealized losses, less than 12 months of temporary impairment	$ (596)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	101
Estimated fair value, less than 12 months of temporary impairment	$ 233,665
Unrealized losses, less than 12 months of temporary impairment	$ (321)
Debt Securities Issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	7
Estimated fair value, less than 12 months of temporary impairment	$ 60,681
Unrealized losses, less than 12 months of temporary impairment	$ (166)
Debt Securities Issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	6
Estimated fair value, less than 12 months of temporary impairment	$ 52,838
Unrealized losses, less than 12 months of temporary impairment	$ (35)
Debt Securities Issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	297
Estimated fair value, less than 12 months of temporary impairment	$ 76,712
Unrealized losses, less than 12 months of temporary impairment	$ (47)
Other Municipal Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	3
Estimated fair value, less than 12 months of temporary impairment	$ 11,343
Unrealized losses, less than 12 months of temporary impairment	$ (27)