Investments (Summary of Gross Unrealized Losses for Investments that have been Continuously in Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Fair Value 12 months or less	$ 297,385	$ 284,888
Unrealized Losses 12 months or less	(6,327)	(3,162)
Fair Value Longer than 12 months	74,769	17,224
Unrealized Losses Longer than 12 months	(2,112)	(254)
Total Fair Value	372,154	302,112
Total Unrealized Losses	(8,439)	(3,416)
Other Investments [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	3,085
Unrealized Losses 12 months or less	(678)
Total Fair Value	3,085
Total Unrealized Losses	(678)
Equity Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	3,017	8,375
Unrealized Losses 12 months or less	(2,654)	(1,504)
Fair Value Longer than 12 months	3,359
Unrealized Losses Longer than 12 months	(736)
Total Fair Value	6,376	8,375
Total Unrealized Losses	(3,390)	(1,504)
Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	291,283	276,513
Unrealized Losses 12 months or less	(2,995)	(1,658)
Fair Value Longer than 12 months	71,410	17,224
Unrealized Losses Longer than 12 months	(1,376)	(254)
Total Fair Value	362,693	293,737
Total Unrealized Losses	(4,371)	(1,912)
U.S. Treasury Securities and Obligations of U.S. Government [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	46,868	28,825
Unrealized Losses 12 months or less	(864)	(145)
Fair Value Longer than 12 months		1,997
Unrealized Losses Longer than 12 months		(3)
Total Fair Value	46,868	30,822
Total Unrealized Losses	(864)	(148)
Corporate Bonds [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	181,467	176,061
Unrealized Losses 12 months or less	(1,313)	(736)
Fair Value Longer than 12 months	47,968	2,378
Unrealized Losses Longer than 12 months	(327)	(6)
Total Fair Value	229,435	178,439
Total Unrealized Losses	(1,640)	(742)
Collateralized Corporate Bank Loans [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	18,794	30,008
Unrealized Losses 12 months or less	(117)	(280)
Fair Value Longer than 12 months	6,064	2,517
Unrealized Losses Longer than 12 months	(23)	(21)
Total Fair Value	24,858	32,525
Total Unrealized Losses	(140)	(301)
Municipal Bonds [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	38,292	35,200
Unrealized Losses 12 months or less	(638)	(370)
Fair Value Longer than 12 months	11,924	8,917
Unrealized Losses Longer than 12 months	(497)	(135)
Total Fair Value	50,216	44,117
Total Unrealized Losses	(1,135)	(505)
Mortgage Backed [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	5,862	6,419
Unrealized Losses 12 months or less	(63)	(127)
Fair Value Longer than 12 months	5,454	1,415
Unrealized Losses Longer than 12 months	(529)	(89)
Total Fair Value	11,316	7,834
Total Unrealized Losses	$ (592)	$ (216)