Investments, Investments Temporarily Impaired (Details) $ in Thousands	Dec. 31, 2018 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	681
Estimated fair value, less than 12 months of temporary impairment	$ 1,358,853
Unrealized losses, less than 12 months of temporary impairment	(2,159)
Estimated fair value, more than 12 months of temporary impairment	225,056
Unrealized losses, more than 12 months of temporary impairment	(1,909)
Estimated fair value, total temporary impairment	1,583,909
Unrealized losses, total temporary impairment	$ (4,068)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	546
Estimated fair value, less than 12 months of temporary impairment	$ 1,000,461
Unrealized losses, less than 12 months of temporary impairment	(1,936)
Estimated fair value, more than 12 months of temporary impairment	126,357
Unrealized losses, more than 12 months of temporary impairment	(1,208)
Estimated fair value, total temporary impairment	1,126,818
Unrealized losses, total temporary impairment	$ (3,144)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	50
Estimated fair value, less than 12 months of temporary impairment	$ 161,312
Unrealized losses, less than 12 months of temporary impairment	(109)
Estimated fair value, more than 12 months of temporary impairment	34,403
Unrealized losses, more than 12 months of temporary impairment	(104)
Estimated fair value, total temporary impairment	195,715
Unrealized losses, total temporary impairment	$ (213)
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	36
Estimated fair value, less than 12 months of temporary impairment	$ 183,212
Unrealized losses, less than 12 months of temporary impairment	(100)
Estimated fair value, more than 12 months of temporary impairment	1,413
Unrealized losses, more than 12 months of temporary impairment	(1)
Estimated fair value, total temporary impairment	184,625
Unrealized losses, total temporary impairment	$ (101)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	49
Estimated fair value, less than 12 months of temporary impairment	$ 13,868
Unrealized losses, less than 12 months of temporary impairment	(14)
Estimated fair value, more than 12 months of temporary impairment	62,883
Unrealized losses, more than 12 months of temporary impairment	(596)
Estimated fair value, total temporary impairment	76,751
Unrealized losses, total temporary impairment	$ (610)