Investments, Investments Temporarily Impaired (Details) $ in Thousands	Sep. 30, 2018 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	626
Estimated fair value, less than 12 months of temporary impairment	$ 1,211,143
Unrealized losses, less than 12 months of temporary impairment	(2,116)
Estimated fair value, more than 12 months of temporary impairment	162,315
Unrealized losses, more than 12 months of temporary impairment	(1,826)
Estimated fair value, total temporary impairment	1,373,458
Unrealized losses, total temporary impairment	$ (3,942)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	495
Estimated fair value, less than 12 months of temporary impairment	$ 889,172
Unrealized losses, less than 12 months of temporary impairment	(1,687)
Estimated fair value, more than 12 months of temporary impairment	83,706
Unrealized losses, more than 12 months of temporary impairment	(997)
Estimated fair value, total temporary impairment	972,878
Unrealized losses, total temporary impairment	$ (2,684)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	51
Estimated fair value, less than 12 months of temporary impairment	$ 132,231
Unrealized losses, less than 12 months of temporary impairment	(139)
Estimated fair value, more than 12 months of temporary impairment	26,862
Unrealized losses, more than 12 months of temporary impairment	(139)
Estimated fair value, total temporary impairment	159,093
Unrealized losses, total temporary impairment	$ (278)
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	32
Estimated fair value, less than 12 months of temporary impairment	$ 164,053
Unrealized losses, less than 12 months of temporary impairment	(99)
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	164,053
Unrealized losses, total temporary impairment	$ (99)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	48
Estimated fair value, less than 12 months of temporary impairment	$ 25,687
Unrealized losses, less than 12 months of temporary impairment	(191)
Estimated fair value, more than 12 months of temporary impairment	51,747
Unrealized losses, more than 12 months of temporary impairment	(690)
Estimated fair value, total temporary impairment	77,434
Unrealized losses, total temporary impairment	$ (881)