Investments (Summary of Gross Unrealized Losses for Investments that have been Continuously in Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Fair Value 12 months or less	$ 411,417	$ 284,888
Unrealized Losses 12 months or less	(7,796)	(3,162)
Fair Value Longer than 12 months	17,536	17,224
Unrealized Losses Longer than 12 months	(1,289)	(254)
Total Fair Value	428,953	302,112
Total Unrealized Losses	(9,085)	(3,416)
Other Investments [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	3,060
Unrealized Losses 12 months or less	(703)
Total Fair Value	3,060
Total Unrealized Losses	(703)
Equity Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	13,106	8,375
Unrealized Losses 12 months or less	(2,990)	(1,504)
Fair Value Longer than 12 months	2,564
Unrealized Losses Longer than 12 months	(850)
Total Fair Value	15,670	8,375
Total Unrealized Losses	(3,840)	(1,504)
Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	395,251	276,513
Unrealized Losses 12 months or less	(4,103)	(1,658)
Fair Value Longer than 12 months	14,972	17,224
Unrealized Losses Longer than 12 months	(439)	(254)
Total Fair Value	410,223	293,737
Total Unrealized Losses	(4,542)	(1,912)
Us Treasury Securities and Obligations of U.S. Government [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	47,133	28,825
Unrealized Losses 12 months or less	(719)	(145)
Fair Value Longer than 12 months		1,997
Unrealized Losses Longer than 12 months		(3)
Total Fair Value	47,133	30,822
Total Unrealized Losses	(719)	(148)
Corporate Bonds [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	227,333	176,061
Unrealized Losses 12 months or less	(1,898)	(736)
Fair Value Longer than 12 months	379	2,378
Unrealized Losses Longer than 12 months	(1)	(6)
Total Fair Value	227,712	178,439
Total Unrealized Losses	(1,899)	(742)
Collateralized Corporate Bank Loans [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	77,457	30,008
Unrealized Losses 12 months or less	(379)	(280)
Fair Value Longer than 12 months	5,687	2,517
Unrealized Losses Longer than 12 months	(91)	(21)
Total Fair Value	83,144	32,525
Total Unrealized Losses	(470)	(301)
Municipal Bonds [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	35,600	35,200
Unrealized Losses 12 months or less	(783)	(370)
Fair Value Longer than 12 months	7,980	8,917
Unrealized Losses Longer than 12 months	(143)	(135)
Total Fair Value	43,580	44,117
Total Unrealized Losses	(926)	(505)
Mortgage Backed [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	7,728	6,419
Unrealized Losses 12 months or less	(324)	(127)
Fair Value Longer than 12 months	926	1,415
Unrealized Losses Longer than 12 months	(204)	(89)
Total Fair Value	8,654	7,834
Total Unrealized Losses	$ (528)	$ (216)