Investments, Investments Temporarily Impaired (Details) $ in Thousands	Mar. 31, 2018 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	529
Estimated fair value, less than 12 months of temporary impairment	$ 676,495
Unrealized losses, less than 12 months of temporary impairment	(3,336)
Estimated fair value, more than 12 months of temporary impairment	129,084
Unrealized losses, more than 12 months of temporary impairment	(1,264)
Estimated fair value, total temporary impairment	805,579
Unrealized losses, total temporary impairment	$ (4,600)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	420
Estimated fair value, less than 12 months of temporary impairment	$ 470,960
Unrealized losses, less than 12 months of temporary impairment	(2,308)
Estimated fair value, more than 12 months of temporary impairment	71,370
Unrealized losses, more than 12 months of temporary impairment	(785)
Estimated fair value, total temporary impairment	542,330
Unrealized losses, total temporary impairment	$ (3,093)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	52
Estimated fair value, less than 12 months of temporary impairment	$ 124,729
Unrealized losses, less than 12 months of temporary impairment	(397)
Estimated fair value, more than 12 months of temporary impairment	25,569
Unrealized losses, more than 12 months of temporary impairment	(88)
Estimated fair value, total temporary impairment	150,298
Unrealized losses, total temporary impairment	$ (485)
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	7
Estimated fair value, less than 12 months of temporary impairment	$ 32,329
Unrealized losses, less than 12 months of temporary impairment	(43)
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	32,329
Unrealized losses, total temporary impairment	$ (43)
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	$ 48,477
Unrealized losses, less than 12 months of temporary impairment	(588)
Estimated fair value, more than 12 months of temporary impairment	32,145
Unrealized losses, more than 12 months of temporary impairment	(391)
Estimated fair value, total temporary impairment	80,622
Unrealized losses, total temporary impairment	$ (979)