Investments, Investments Temporarily Impaired (Details) $ in Thousands	Mar. 31, 2017 USD ($) Investment
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	496
Estimated fair value, less than 12 months of temporary impairment	$ 621,894
Unrealized losses, less than 12 months of temporary impairment	(1,732)
Estimated fair value, more than 12 months of temporary impairment	27,303
Unrealized losses, more than 12 months of temporary impairment	(129)
Estimated fair value, total temporary impairment	649,197
Unrealized losses, total temporary impairment	$ (1,861)
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	354
Estimated fair value, less than 12 months of temporary impairment	$ 421,288
Unrealized losses, less than 12 months of temporary impairment	(1,180)
Estimated fair value, more than 12 months of temporary impairment	22,387
Unrealized losses, more than 12 months of temporary impairment	(58)
Estimated fair value, total temporary impairment	443,675
Unrealized losses, total temporary impairment	$ (1,238)
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	41
Estimated fair value, less than 12 months of temporary impairment	$ 84,017
Unrealized losses, less than 12 months of temporary impairment	(196)
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	84,017
Unrealized losses, total temporary impairment	$ (196)
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	$ 25,773
Unrealized losses, less than 12 months of temporary impairment	(20)
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	25,773
Unrealized losses, total temporary impairment	$ (20)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment	97
Estimated fair value, less than 12 months of temporary impairment	$ 90,816
Unrealized losses, less than 12 months of temporary impairment	(336)
Estimated fair value, more than 12 months of temporary impairment	4,916
Unrealized losses, more than 12 months of temporary impairment	(71)
Estimated fair value, total temporary impairment	95,732
Unrealized losses, total temporary impairment	$ (407)