Investments, Investments Temporarily Impaired (Details) $ in Thousands	Sep. 30, 2020 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	110
Estimated fair value, less than 12 months of temporary impairment	$ 218,960
Unrealized losses, less than 12 months of temporary impairment	(170)
Estimated fair value, more than 12 months of temporary impairment	7,000
Unrealized losses, more than 12 months of temporary impairment	0
Estimated fair value, total temporary impairment	225,960
Unrealized losses, total temporary impairment	$ (170)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	54
Estimated fair value, less than 12 months of temporary impairment	$ 104,736
Unrealized losses, less than 12 months of temporary impairment	(99)
Estimated fair value, more than 12 months of temporary impairment	0
Unrealized losses, more than 12 months of temporary impairment	0
Estimated fair value, total temporary impairment	104,736
Unrealized losses, total temporary impairment	$ (99)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	4
Estimated fair value, less than 12 months of temporary impairment	$ 37,963
Unrealized losses, less than 12 months of temporary impairment	(32)
Estimated fair value, more than 12 months of temporary impairment	7,000
Unrealized losses, more than 12 months of temporary impairment	0
Estimated fair value, total temporary impairment	44,963
Unrealized losses, total temporary impairment	$ (32)
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	5
Estimated fair value, less than 12 months of temporary impairment	$ 55,273
Unrealized losses, less than 12 months of temporary impairment	(13)
Estimated fair value, more than 12 months of temporary impairment	0
Unrealized losses, more than 12 months of temporary impairment	0
Estimated fair value, total temporary impairment	55,273
Unrealized losses, total temporary impairment	$ (13)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	45
Estimated fair value, less than 12 months of temporary impairment	$ 14,719
Unrealized losses, less than 12 months of temporary impairment	(3)
Estimated fair value, more than 12 months of temporary impairment	0
Unrealized losses, more than 12 months of temporary impairment	0
Estimated fair value, total temporary impairment	14,719
Unrealized losses, total temporary impairment	$ (3)
Other Municipal Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	2
Estimated fair value, less than 12 months of temporary impairment	$ 6,269
Unrealized losses, less than 12 months of temporary impairment	(23)
Estimated fair value, more than 12 months of temporary impairment	0
Unrealized losses, more than 12 months of temporary impairment	0
Estimated fair value, total temporary impairment	6,269
Unrealized losses, total temporary impairment	$ (23)