Investments (Summary of Gross Unrealized Loss Position) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair Value 12 months or less	$ 284,888	$ 200,483
Unrealized Losses 12 months or less	(3,162)	(2,760)
Fair Value Longer than 12 months	17,224	26,355
Unrealized Losses Longer than 12 months	(254)	(2,109)
Fair Value Total	302,112	226,838
Unrealized Losses Total	(3,416)	(4,869)
Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value 12 months or less
Unrealized Losses 12 months or less
Fair Value Longer than 12 months
Unrealized Losses Longer than 12 months
Fair Value Total
Unrealized Losses Total
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value 12 months or less	276,513	196,374
Unrealized Losses 12 months or less	(1,658)	(2,277)
Fair Value Longer than 12 months	17,224	24,318
Unrealized Losses Longer than 12 months	(254)	(1,802)
Fair Value Total	293,737	220,692
Unrealized Losses Total	(1,912)	(4,079)
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value 12 months or less	8,375	4,109
Unrealized Losses 12 months or less	(1,504)	(483)
Fair Value Longer than 12 months		2,037
Unrealized Losses Longer than 12 months		(307)
Fair Value Total	8,375	6,146
Unrealized Losses Total	(1,504)	(790)
U.S. Treasury securities and obligations of U.S. Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value 12 months or less	28,825	7,037
Unrealized Losses 12 months or less	(145)	(20)
Fair Value Longer than 12 months	1,997
Unrealized Losses Longer than 12 months	(3)
Fair Value Total	30,822	7,037
Unrealized Losses Total	(148)	(20)
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value 12 months or less	176,061	86,592
Unrealized Losses 12 months or less	(736)	(575)
Fair Value Longer than 12 months	2,378
Unrealized Losses Longer than 12 months	(6)
Fair Value Total	178,439	86,592
Unrealized Losses Total	(742)	(575)
Collateralized Corporate Bank Loans [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value 12 months or less	30,008	2,637
Unrealized Losses 12 months or less	(280)	(7)
Fair Value Longer than 12 months	2,517	8,314
Unrealized Losses Longer than 12 months	(21)	(163)
Fair Value Total	32,525	10,951
Unrealized Losses Total	(301)	(170)
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value 12 months or less	35,200	70,633
Unrealized Losses 12 months or less	(370)	(1,327)
Fair Value Longer than 12 months	8,917	13,574
Unrealized Losses Longer than 12 months	(135)	(1,634)
Fair Value Total	44,117	84,207
Unrealized Losses Total	(505)	(2,961)
Collateralized Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value 12 months or less	6,419	29,475
Unrealized Losses 12 months or less	(127)	(348)
Fair Value Longer than 12 months	1,415	2,430
Unrealized Losses Longer than 12 months	(89)	(5)
Fair Value Total	7,834	31,905
Unrealized Losses Total	$ (216)	$ (353)