Investments (Amortized Cost and Estimated Fair Value of Investments in Debt and Equity Securities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	$ 632,996	$ 563,580
Investments, Gross Unrealized Gains	25,992	26,122
Investments, Gross Unrealized Losses	(4,869)	(10,873)
Total investments	654,119	578,829
Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	3,763	427
Investments, Gross Unrealized Gains	1,188	27
Total investments	4,951	454
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	31,449	24,524
Investments, Gross Unrealized Gains	21,052	23,364
Investments, Gross Unrealized Losses	(790)	(838)
Total investments	51,711	47,050
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	597,784	538,629
Investments, Gross Unrealized Gains	3,752	2,731
Investments, Gross Unrealized Losses	(4,079)	(10,035)
Total investments	597,457	531,325
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	41,976	76,323
Investments, Gross Unrealized Gains	66	7
Investments, Gross Unrealized Losses	(20)	(61)
Total investments	42,022	76,269
Corporate Bonds [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	224,915	122,894
Investments, Gross Unrealized Gains	1,722	637
Investments, Gross Unrealized Losses	(575)	(1,822)
Total investments	226,062	121,709
Collateralized Corporate Bank Loans [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	105,220	83,434
Investments, Gross Unrealized Gains	959	44
Investments, Gross Unrealized Losses	(170)	(1,882)
Total investments	106,009	81,596
Municipal Bonds [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	165,900	196,446
Investments, Gross Unrealized Gains	956	1,888
Investments, Gross Unrealized Losses	(2,961)	(5,966)
Total investments	163,895	192,368
Collateralized Mortgage Backed Securities [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	59,773	59,532
Investments, Gross Unrealized Gains	49	155
Investments, Gross Unrealized Losses	(353)	(304)
Total investments	59,469	$ 59,383
Equity Securities All Others [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Gross Unrealized Gains	$ 1,188