Investments, Investments Temporarily Impaired (Details) $ in Thousands	Dec. 31, 2016 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	421
Estimated fair value, less than 12 months of temporary impairment	$ 423,922
Unrealized losses, less than 12 months of temporary impairment	(1,786)
Estimated fair value, more than 12 months of temporary impairment	39,687
Unrealized losses, more than 12 months of temporary impairment	(223)
Estimated fair value, total temporary impairment	463,609
Unrealized losses, total temporary impairment	$ (2,009)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	255
Estimated fair value, less than 12 months of temporary impairment	$ 280,393
Unrealized losses, less than 12 months of temporary impairment	(1,197)
Estimated fair value, more than 12 months of temporary impairment	32,753
Unrealized losses, more than 12 months of temporary impairment	(138)
Estimated fair value, total temporary impairment	313,146
Unrealized losses, total temporary impairment	$ (1,335)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	29
Estimated fair value, less than 12 months of temporary impairment	$ 43,851
Unrealized losses, less than 12 months of temporary impairment	(177)
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	43,851
Unrealized losses, total temporary impairment	$ (177)
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	2
Estimated fair value, less than 12 months of temporary impairment	$ 18,782
Unrealized losses, less than 12 months of temporary impairment	(14)
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	18,782
Unrealized losses, total temporary impairment	$ (14)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	135
Estimated fair value, less than 12 months of temporary impairment	$ 80,896
Unrealized losses, less than 12 months of temporary impairment	(398)
Estimated fair value, more than 12 months of temporary impairment	6,934
Unrealized losses, more than 12 months of temporary impairment	(85)
Estimated fair value, total temporary impairment	87,830
Unrealized losses, total temporary impairment	$ (483)