Investments, Investments Temporarily Impaired (Details) $ in Thousands	Sep. 30, 2019 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	154
Estimated fair value, less than 12 months of temporary impairment	$ 486,854
Unrealized losses, less than 12 months of temporary impairment	(480)
Estimated fair value, more than 12 months of temporary impairment	70,246
Unrealized losses, more than 12 months of temporary impairment	(99)
Estimated fair value, total temporary impairment	557,100
Unrealized losses, total temporary impairment	$ (579)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	104
Estimated fair value, less than 12 months of temporary impairment	$ 304,705
Unrealized losses, less than 12 months of temporary impairment	(268)
Estimated fair value, more than 12 months of temporary impairment	31,675
Unrealized losses, more than 12 months of temporary impairment	(41)
Estimated fair value, total temporary impairment	336,380
Unrealized losses, total temporary impairment	$ (309)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	28
Estimated fair value, less than 12 months of temporary impairment	$ 102,733
Unrealized losses, less than 12 months of temporary impairment	(156)
Estimated fair value, more than 12 months of temporary impairment	22,974
Unrealized losses, more than 12 months of temporary impairment	(26)
Estimated fair value, total temporary impairment	125,707
Unrealized losses, total temporary impairment	$ (182)
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	9
Estimated fair value, less than 12 months of temporary impairment	$ 77,918
Unrealized losses, less than 12 months of temporary impairment	(55)
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	77,918
Unrealized losses, total temporary impairment	$ (55)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	13
Estimated fair value, less than 12 months of temporary impairment	$ 1,498
Unrealized losses, less than 12 months of temporary impairment	(1)
Estimated fair value, more than 12 months of temporary impairment	15,597
Unrealized losses, more than 12 months of temporary impairment	(32)
Estimated fair value, total temporary impairment	17,095
Unrealized losses, total temporary impairment	$ (33)