Investments (Amortized Cost and Estimated Fair Value of Investments in Debt and Equity Securities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	$ 622,740	$ 639,015
Investments, Gross Unrealized Gains	24,959	25,734
Investments, Gross Unrealized Losses	(19,785)	(3,416)
Total investments	627,914	661,333
Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	3,763	3,763
Investments, Gross Unrealized Gains		61
Investments, Gross Unrealized Losses	(2,615)
Total investments	1,148	3,824
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	68,709	30,253
Investments, Gross Unrealized Gains	20,693	23,014
Investments, Gross Unrealized Losses	(8,506)	(1,504)
Total investments	80,896	51,763
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	550,268	604,999
Investments, Gross Unrealized Gains	4,266	2,659
Investments, Gross Unrealized Losses	(8,664)	(1,912)
Total investments	545,870	605,746
U.S. Treasury securities and obligations of U.S. Government [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	48,609	50,088
Investments, Gross Unrealized Gains	5	7
Investments, Gross Unrealized Losses	(508)	(148)
Total investments	48,106	49,947
Corporate Bonds [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	243,314	278,611
Investments, Gross Unrealized Gains	440	1,204
Investments, Gross Unrealized Losses	(1,602)	(742)
Total investments	242,152	279,073
Collateralized Corporate Bank Loans [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	131,779	125,536
Investments, Gross Unrealized Gains	19	702
Investments, Gross Unrealized Losses	(5,270)	(301)
Total investments	126,528	125,937
Municipal Bonds [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	112,574	134,052
Investments, Gross Unrealized Gains	3,791	709
Investments, Gross Unrealized Losses	(838)	(505)
Total investments	115,527	134,256
Collateralized Mortgage Backed Securities [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	13,992	16,712
Investments, Gross Unrealized Gains	11	37
Investments, Gross Unrealized Losses	(446)	(216)
Total investments	13,557	16,533
Equity Securities All Others [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Gross Unrealized Losses	$ (2,676)	$ (1,127)