Investments, Investments Temporarily Impaired (Details) $ in Thousands	Mar. 31, 2020 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	320
Estimated fair value, less than 12 months of temporary impairment	$ 817,109
Unrealized losses, less than 12 months of temporary impairment	(4,829)
Estimated fair value, more than 12 months of temporary impairment	34,053
Unrealized losses, more than 12 months of temporary impairment	(11)
Estimated fair value, total temporary impairment	851,162
Unrealized losses, total temporary impairment	$ (4,840)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	305
Estimated fair value, less than 12 months of temporary impairment	$ 750,404
Unrealized losses, less than 12 months of temporary impairment	(4,793)
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	750,404
Unrealized losses, total temporary impairment	$ (4,793)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	11
Estimated fair value, less than 12 months of temporary impairment	$ 47,473
Unrealized losses, less than 12 months of temporary impairment	(32)
Estimated fair value, more than 12 months of temporary impairment	34,053
Unrealized losses, more than 12 months of temporary impairment	(11)
Estimated fair value, total temporary impairment	81,526
Unrealized losses, total temporary impairment	$ (43)
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	1
Estimated fair value, less than 12 months of temporary impairment	$ 15,598
Unrealized losses, less than 12 months of temporary impairment	(2)
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	15,598
Unrealized losses, total temporary impairment	$ (2)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	3
Estimated fair value, less than 12 months of temporary impairment	$ 3,634
Unrealized losses, less than 12 months of temporary impairment	(2)
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	3,634
Unrealized losses, total temporary impairment	$ (2)