Investments (Summary of Gross Unrealized Losses for Investments that have been Continuously in Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Fair Value 12 months or less	$ 157,773	$ 309,764
Unrealized Losses 12 months or less	(5,202)	(12,496)
Fair Value Longer than 12 months	177,290	169,544
Unrealized Losses Longer than 12 months	(5,975)	(7,289)
Total Fair Value	335,063	479,308
Total Unrealized Losses	(11,177)	(19,785)
Other Investments [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	914	1,148
Unrealized Losses 12 months or less	(1,873)	(2,615)
Fair Value Longer than 12 months	270
Unrealized Losses Longer than 12 months	(706)
Total Fair Value	1,184	1,148
Total Unrealized Losses	(2,579)	(2,615)
Equity Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	16,082	30,981
Unrealized Losses 12 months or less	(1,518)	(3,699)
Fair Value Longer than 12 months	5,537	4,475
Unrealized Losses Longer than 12 months	(4,084)	(4,807)
Total Fair Value	21,619	35,456
Total Unrealized Losses	(5,602)	(8,506)
Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	140,777	277,635
Unrealized Losses 12 months or less	(1,811)	(6,182)
Fair Value Longer than 12 months	171,483	165,069
Unrealized Losses Longer than 12 months	(1,185)	(2,482)
Total Fair Value	312,260	442,704
Total Unrealized Losses	(2,996)	(8,664)
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		18,902
Unrealized Losses 12 months or less		(181)
Fair Value Longer than 12 months	47,205	28,201
Unrealized Losses Longer than 12 months	(288)	(327)
Total Fair Value	47,205	47,103
Total Unrealized Losses	(288)	(508)
Corporate Bonds [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	13,462	117,450
Unrealized Losses 12 months or less	(72)	(907)
Fair Value Longer than 12 months	85,392	100,060
Unrealized Losses Longer than 12 months	(368)	(695)
Total Fair Value	98,854	217,510
Total Unrealized Losses	(440)	(1,602)
Collateralized Corporate Bank Loans [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	115,083	120,410
Unrealized Losses 12 months or less	(1,660)	(4,938)
Fair Value Longer than 12 months	4,832	4,931
Unrealized Losses Longer than 12 months	(92)	(332)
Total Fair Value	119,915	125,341
Total Unrealized Losses	(1,752)	(5,270)
Municipal Bonds [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	11,463	14,281
Unrealized Losses 12 months or less	(79)	(96)
Fair Value Longer than 12 months	25,229	25,891
Unrealized Losses Longer than 12 months	(219)	(742)
Total Fair Value	36,692	40,172
Total Unrealized Losses	(298)	(838)
Mortgage Backed [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	769	6,592
Unrealized Losses 12 months or less		(60)
Fair Value Longer than 12 months	8,825	5,986
Unrealized Losses Longer than 12 months	(218)	(386)
Total Fair Value	9,594	12,578
Total Unrealized Losses	$ (218)	$ (446)