Investments, Investments Temporarily Impaired (Details) $ in Thousands	Jun. 30, 2019 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	185
Estimated fair value, less than 12 months of temporary impairment	$ 476,280
Unrealized losses, less than 12 months of temporary impairment	(434)
Estimated fair value, more than 12 months of temporary impairment	106,806
Unrealized losses, more than 12 months of temporary impairment	(278)
Estimated fair value, total temporary impairment	583,086
Unrealized losses, total temporary impairment	$ (712)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	145
Estimated fair value, less than 12 months of temporary impairment	$ 340,153
Unrealized losses, less than 12 months of temporary impairment	(302)
Estimated fair value, more than 12 months of temporary impairment	63,109
Unrealized losses, more than 12 months of temporary impairment	(157)
Estimated fair value, total temporary impairment	403,262
Unrealized losses, total temporary impairment	$ (459)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	14
Estimated fair value, less than 12 months of temporary impairment	$ 89,023
Unrealized losses, less than 12 months of temporary impairment	(105)
Estimated fair value, more than 12 months of temporary impairment	2,980
Unrealized losses, more than 12 months of temporary impairment	(18)
Estimated fair value, total temporary impairment	92,003
Unrealized losses, total temporary impairment	$ (123)
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	3
Estimated fair value, less than 12 months of temporary impairment	$ 47,104
Unrealized losses, less than 12 months of temporary impairment	(27)
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	47,104
Unrealized losses, total temporary impairment	$ (27)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	23
Estimated fair value, less than 12 months of temporary impairment	$ 0
Unrealized losses, less than 12 months of temporary impairment	0
Estimated fair value, more than 12 months of temporary impairment	40,717
Unrealized losses, more than 12 months of temporary impairment	(103)
Estimated fair value, total temporary impairment	40,717
Unrealized losses, total temporary impairment	$ (103)