Investments, Investments Temporarily Impaired (Details) $ in Thousands	Mar. 31, 2019 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	353
Estimated fair value, less than 12 months of temporary impairment	$ 727,648
Unrealized losses, less than 12 months of temporary impairment	(430)
Estimated fair value, more than 12 months of temporary impairment	170,928
Unrealized losses, more than 12 months of temporary impairment	(926)
Estimated fair value, total temporary impairment	898,576
Unrealized losses, total temporary impairment	$ (1,356)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	275
Estimated fair value, less than 12 months of temporary impairment	$ 530,786
Unrealized losses, less than 12 months of temporary impairment	(306)
Estimated fair value, more than 12 months of temporary impairment	102,815
Unrealized losses, more than 12 months of temporary impairment	(561)
Estimated fair value, total temporary impairment	633,601
Unrealized losses, total temporary impairment	$ (867)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	24
Estimated fair value, less than 12 months of temporary impairment	$ 91,005
Unrealized losses, less than 12 months of temporary impairment	(81)
Estimated fair value, more than 12 months of temporary impairment	16,534
Unrealized losses, more than 12 months of temporary impairment	(46)
Estimated fair value, total temporary impairment	107,539
Unrealized losses, total temporary impairment	$ (127)
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	15
Estimated fair value, less than 12 months of temporary impairment	$ 94,603
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	94,603
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	39
Estimated fair value, less than 12 months of temporary impairment	$ 11,254
Unrealized losses, less than 12 months of temporary impairment	(9)
Estimated fair value, more than 12 months of temporary impairment	51,579
Unrealized losses, more than 12 months of temporary impairment	(319)
Estimated fair value, total temporary impairment	62,833
Unrealized losses, total temporary impairment	$ (328)