Investments, Investments Temporarily Impaired (Details) $ in Thousands	Sep. 30, 2016 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	298
Estimated fair value, less than 12 months of temporary impairment	$ 298,425
Unrealized losses, less than 12 months of temporary impairment	(643)
Estimated fair value, more than 12 months of temporary impairment	50,618
Unrealized losses, more than 12 months of temporary impairment	(474)
Estimated fair value, total temporary impairment	349,043
Unrealized losses, total temporary impairment	$ (1,117)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	162
Estimated fair value, less than 12 months of temporary impairment	$ 203,357
Unrealized losses, less than 12 months of temporary impairment	(470)
Estimated fair value, more than 12 months of temporary impairment	33,639
Unrealized losses, more than 12 months of temporary impairment	(395)
Estimated fair value, total temporary impairment	236,996
Unrealized losses, total temporary impairment	$ (865)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	19
Estimated fair value, less than 12 months of temporary impairment	$ 31,517
Unrealized losses, less than 12 months of temporary impairment	(33)
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	31,517
Unrealized losses, total temporary impairment	$ (33)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	117
Estimated fair value, less than 12 months of temporary impairment	$ 63,551
Unrealized losses, less than 12 months of temporary impairment	(140)
Estimated fair value, more than 12 months of temporary impairment	16,979
Unrealized losses, more than 12 months of temporary impairment	(79)
Estimated fair value, total temporary impairment	80,530
Unrealized losses, total temporary impairment	$ (219)