Marketable Securities (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Realized Gain (Loss) on Marketable Securities and Cost Method Investments, Excluding Other than Temporary Impairments, and Other Investments	$ 0
Amortized Cost Basis	794,490,000	$ 856,429,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	12,216,000	14,622,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(6,531,000)	(2,769,000)
Available-for-sale Securities	800,175,000	868,282,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	244,965,000	236,121,000
Maturities after one year through five years	533,795,000	600,695,000
Total	778,760,000	836,816,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	151,831,000	731,331,000
Fair Value - 12 Months or Greater	135,211,000	23,749,000
Fair Value - Total	287,042,000	755,080,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(5,842,000)	(2,677,000)
Unrealized Loss - 12 Months or Greater	(689,000)	(92,000)
Unrealized Loss - Total	(6,531,000)	(2,769,000)
Realized Gain (Loss) on Marketable Securities, Cost Method Investments, and Other Investments	0
Corporate Bond Securities [Member]
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	132,085,000	668,199,000
Fair Value - 12 Months or Greater	135,211,000	23,749,000
Fair Value - Total	267,296,000	691,948,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(190,000)	(2,473,000)
Unrealized Loss - 12 Months or Greater	(689,000)	(92,000)
Unrealized Loss - Total	(879,000)	(2,565,000)
U.S. government and government agency obligations
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	7,822,000	51,215,000
Fair Value - 12 Months or Greater	0	0
Fair Value - Total	7,822,000	51,215,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(3,000)	(193,000)
Unrealized Loss - 12 Months or Greater	0	0
Unrealized Loss - Total	(3,000)	(193,000)
Municipal bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	2,571,000	11,917,000
Fair Value - 12 Months or Greater	0	0
Fair Value - Total	2,571,000	11,917,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(2,000)	(11,000)
Unrealized Loss - 12 Months or Greater	0	0
Unrealized Loss - Total	(2,000)	(11,000)
Equity Securities
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	9,353,000
Fair Value - 12 Months or Greater	0
Fair Value - Total	9,353,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(5,647,000)
Unrealized Loss - 12 Months or Greater	0
Unrealized Loss - Total	(5,647,000)
Unrestricted [Member] | Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	710,989,000	770,092,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,920,000	156,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(879,000)	(2,565,000)
Available-for-sale Securities	712,030,000	767,683,000
Unrestricted [Member] | U.S. government and government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	50,383,000	51,402,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	194,000	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(2,000)	(193,000)
Available-for-sale Securities	50,575,000	51,209,000
Unrestricted [Member] | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	16,113,000	17,930,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	45,000	5,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(3,000)	(11,000)
Available-for-sale Securities	16,155,000	17,924,000
Unrestricted [Member] | Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	17,005,000	17,005,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	10,057,000	14,461,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(5,647,000)	0
Available-for-sale Securities	$ 21,415,000	$ 31,466,000