Investments, Investments Temporarily Impaired (Details) $ in Thousands	Jun. 30, 2018 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	645
Estimated fair value, less than 12 months of temporary impairment	$ 1,054,430
Unrealized losses, less than 12 months of temporary impairment	(2,904)
Estimated fair value, more than 12 months of temporary impairment	115,024
Unrealized losses, more than 12 months of temporary impairment	(1,191)
Estimated fair value, total temporary impairment	1,169,454
Unrealized losses, total temporary impairment	$ (4,095)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	543
Estimated fair value, less than 12 months of temporary impairment	$ 841,374
Unrealized losses, less than 12 months of temporary impairment	(2,060)
Estimated fair value, more than 12 months of temporary impairment	61,915
Unrealized losses, more than 12 months of temporary impairment	(757)
Estimated fair value, total temporary impairment	903,289
Unrealized losses, total temporary impairment	$ (2,817)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	44
Estimated fair value, less than 12 months of temporary impairment	$ 106,489
Unrealized losses, less than 12 months of temporary impairment	(296)
Estimated fair value, more than 12 months of temporary impairment	19,141
Unrealized losses, more than 12 months of temporary impairment	(9)
Estimated fair value, total temporary impairment	125,630
Unrealized losses, total temporary impairment	$ (305)
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	8
Estimated fair value, less than 12 months of temporary impairment	$ 63,634
Unrealized losses, less than 12 months of temporary impairment	(34)
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	63,634
Unrealized losses, total temporary impairment	$ (34)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	50
Estimated fair value, less than 12 months of temporary impairment	$ 42,933
Unrealized losses, less than 12 months of temporary impairment	(514)
Estimated fair value, more than 12 months of temporary impairment	33,968
Unrealized losses, more than 12 months of temporary impairment	(425)
Estimated fair value, total temporary impairment	76,901
Unrealized losses, total temporary impairment	$ (939)