Investments, Investments Temporarily Impaired (Details) $ in Thousands	Jun. 30, 2016 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	195
Estimated fair value, less than 12 months of temporary impairment	$ 145,502
Unrealized losses, less than 12 months of temporary impairment	(941)
Estimated fair value, more than 12 months of temporary impairment	62,423
Unrealized losses, more than 12 months of temporary impairment	(258)
Estimated fair value, total temporary impairment	207,925
Unrealized losses, total temporary impairment	$ (1,199)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	103
Estimated fair value, less than 12 months of temporary impairment	$ 96,050
Unrealized losses, less than 12 months of temporary impairment	(65)
Estimated fair value, more than 12 months of temporary impairment	43,526
Unrealized losses, more than 12 months of temporary impairment	(182)
Estimated fair value, total temporary impairment	139,576
Unrealized losses, total temporary impairment	$ (247)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	2
Estimated fair value, less than 12 months of temporary impairment	$ 4,999
Unrealized losses, less than 12 months of temporary impairment	(1)
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	4,999
Unrealized losses, total temporary impairment	$ (1)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	89
Estimated fair value, less than 12 months of temporary impairment	$ 42,285
Unrealized losses, less than 12 months of temporary impairment	(42)
Estimated fair value, more than 12 months of temporary impairment	18,897
Unrealized losses, more than 12 months of temporary impairment	(76)
Estimated fair value, total temporary impairment	61,182
Unrealized losses, total temporary impairment	$ (118)
Equity Securities [Member] | Regulus Therapeutics Inc. [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	1
Estimated fair value, less than 12 months of temporary impairment	$ 2,168
Unrealized losses, less than 12 months of temporary impairment	(833)
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	2,168
Unrealized losses, total temporary impairment	$ (833)