Investments (Summary of Gross Unrealized Losses for Investments that have been Continuously in Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Fair Value 12 months or less	$ 341,950	$ 284,888
Unrealized Losses 12 months or less	(7,674)	(3,162)
Fair Value Longer than 12 months	26,574	17,224
Unrealized Losses Longer than 12 months	(613)	(254)
Total Fair Value	368,524	302,112
Total Unrealized Losses	(8,287)	(3,416)
Other Investments [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	3,461
Unrealized Losses 12 months or less	(302)
Total Fair Value	3,461
Total Unrealized Losses	(302)
Equity Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	12,444	8,375
Unrealized Losses 12 months or less	(4,185)	(1,504)
Total Fair Value	12,444	8,375
Total Unrealized Losses	(4,185)	(1,504)
Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	326,045	276,513
Unrealized Losses 12 months or less	(3,187)	(1,658)
Fair Value Longer than 12 months	26,574	17,224
Unrealized Losses Longer than 12 months	(613)	(254)
Total Fair Value	352,619	293,737
Total Unrealized Losses	(3,800)	(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	47,418	28,825
Unrealized Losses 12 months or less	(553)	(145)
Fair Value Longer than 12 months	1,999	1,997
Unrealized Losses Longer than 12 months	(1)	(3)
Total Fair Value	49,417	30,822
Total Unrealized Losses	(554)	(148)
Corporate Bonds [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	218,114	176,061
Unrealized Losses 12 months or less	(1,861)	(736)
Fair Value Longer than 12 months	1,385	2,378
Unrealized Losses Longer than 12 months	(32)	(6)
Total Fair Value	219,499	178,439
Total Unrealized Losses	(1,893)	(742)
Collateralized Corporate Bank Loans [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	16,828	30,008
Unrealized Losses 12 months or less	(43)	(280)
Fair Value Longer than 12 months	8,006	2,517
Unrealized Losses Longer than 12 months	(90)	(21)
Total Fair Value	24,834	32,525
Total Unrealized Losses	(133)	(301)
Municipal Bonds [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	38,925	35,200
Unrealized Losses 12 months or less	(607)	(370)
Fair Value Longer than 12 months	10,232	8,917
Unrealized Losses Longer than 12 months	(214)	(135)
Total Fair Value	49,157	44,117
Total Unrealized Losses	(821)	(505)
Mortgage Backed [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value 12 months or less	4,760	6,419
Unrealized Losses 12 months or less	(123)	(127)
Fair Value Longer than 12 months	4,952	1,415
Unrealized Losses Longer than 12 months	(276)	(89)
Total Fair Value	9,712	7,834
Total Unrealized Losses	$ (399)	$ (216)