Investments, Investments Temporarily Impaired (Details) $ in Thousands	Jun. 30, 2017 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	475
Estimated fair value, less than 12 months of temporary impairment	$ 616,847
Unrealized losses, less than 12 months of temporary impairment	(1,619)
Estimated fair value, more than 12 months of temporary impairment	20,884
Unrealized losses, more than 12 months of temporary impairment	(106)
Estimated fair value, total temporary impairment	637,731
Unrealized losses, total temporary impairment	$ (1,725)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	354
Estimated fair value, less than 12 months of temporary impairment	$ 423,899
Unrealized losses, less than 12 months of temporary impairment	(1,001)
Estimated fair value, more than 12 months of temporary impairment	18,197
Unrealized losses, more than 12 months of temporary impairment	(33)
Estimated fair value, total temporary impairment	442,096
Unrealized losses, total temporary impairment	$ (1,034)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	42
Estimated fair value, less than 12 months of temporary impairment	$ 81,100
Unrealized losses, less than 12 months of temporary impairment	(206)
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	81,100
Unrealized losses, total temporary impairment	$ (206)
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	4
Estimated fair value, less than 12 months of temporary impairment	$ 25,772
Unrealized losses, less than 12 months of temporary impairment	(25)
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	25,772
Unrealized losses, total temporary impairment	$ (25)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	74
Estimated fair value, less than 12 months of temporary impairment	$ 83,078
Unrealized losses, less than 12 months of temporary impairment	(385)
Estimated fair value, more than 12 months of temporary impairment	2,687
Unrealized losses, more than 12 months of temporary impairment	(73)
Estimated fair value, total temporary impairment	85,765
Unrealized losses, total temporary impairment	$ (458)
Other Municipal Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	1
Estimated fair value, less than 12 months of temporary impairment	$ 2,998
Unrealized losses, less than 12 months of temporary impairment	(2)
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	2,998
Unrealized losses, total temporary impairment	$ (2)