Investments (Amortized Cost and Estimated Fair Value of Investments in Debt and Equity Securities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	$ 639,015	$ 632,996
Investments, Gross Unrealized Gains	25,734	25,992
Investments, Gross Unrealized Losses	(3,416)	(4,869)
Total investments	661,333	654,119
Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	3,763	3,763
Investments, Gross Unrealized Gains	61	1,188
Total investments	3,824	4,951
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	30,253	31,449
Investments, Gross Unrealized Gains	23,014	21,052
Investments, Gross Unrealized Losses	(1,504)	(790)
Total investments	51,763	51,711
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	604,999	597,784
Investments, Gross Unrealized Gains	2,659	3,752
Investments, Gross Unrealized Losses	(1,912)	(4,079)
Total investments	605,746	597,457
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	50,088	41,976
Investments, Gross Unrealized Gains	7	66
Investments, Gross Unrealized Losses	(148)	(20)
Total investments	49,947	42,022
Corporate Bonds [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	278,611	224,915
Investments, Gross Unrealized Gains	1,204	1,722
Investments, Gross Unrealized Losses	(742)	(575)
Total investments	279,073	226,062
Collateralized Corporate Bank Loans [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	125,536	105,220
Investments, Gross Unrealized Gains	702	959
Investments, Gross Unrealized Losses	(301)	(170)
Total investments	125,937	106,009
Municipal Bonds [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	134,052	165,900
Investments, Gross Unrealized Gains	709	956
Investments, Gross Unrealized Losses	(505)	(2,961)
Total investments	134,256	163,895
Collateralized Mortgage Backed Securities [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	16,712	59,773
Investments, Gross Unrealized Gains	37	49
Investments, Gross Unrealized Losses	(216)	(353)
Total investments	16,533	59,469
Equity Securities All Others [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Gross Unrealized Gains		$ 1,188
Investments, Gross Unrealized Losses	$ (1,127)