Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains		$ 1,200	$ 1,000
Amortized Cost Basis	$ 856,429	631,477
Unrealized Gains	14,622	81,989
Unrealized (Losses)	(2,769)	(1,551)
Total fair value of available-for-sale marketable securities	868,282	711,915
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	236,121	251,761
Maturities after one year through five years	600,695	360,208
Maturities after five years through ten years	0	1,128
Total	836,816	613,097
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	731,331	426,941
Fair Value - 12 Months or Greater	23,749	0
Fair Value - Total	755,080	426,941
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(2,677)	(1,551)
Unrealized Loss- 12 Months or Greater	(92)	0
Unrealized Loss - Total	(2,769)	(1,551)
Available-for-sale Securities, Gross Realized Losses		0	0
Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	548,322	616,477
Unrealized Gains	392	5,550
Unrealized (Losses)	(447)	(1,551)
Total fair value of available-for-sale marketable securities	548,267	620,476
U.S. government and government agency obligations
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	51,215	25,549
Fair Value - 12 Months or Greater	0	0
Fair Value - Total	51,215	25,549
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(193)	(46)
Unrealized Loss- 12 Months or Greater	0	0
Unrealized Loss - Total	(193)	(46)
U.S. government and government agency obligations | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	51,402	28,596
Unrealized Gains	0	3
Unrealized (Losses)	(193)	(46)
Total fair value of available-for-sale marketable securities	51,209	28,553
Corporate bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	668,199	390,613
Fair Value - 12 Months or Greater	23,749	0
Fair Value - Total	691,948	390,613
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(2,473)	(1,462)
Unrealized Loss- 12 Months or Greater	(92)	0
Unrealized Loss - Total	(2,565)	(1,462)
Corporate bonds | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	770,092	548,832
Unrealized Gains	156	136
Unrealized (Losses)	(2,565)	(1,462)
Total fair value of available-for-sale marketable securities	767,683	547,506
Municipal Bonds [Member]
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	11,917	10,779
Fair Value - 12 Months or Greater	0	0
Fair Value - Total	11,917	10,779
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(11)	(43)
Unrealized Loss- 12 Months or Greater	0	0
Unrealized Loss - Total	(11)	(43)
Municipal Bonds [Member] | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	17,930	37,044
Unrealized Gains	5	37
Unrealized (Losses)	(11)	(43)
Total fair value of available-for-sale marketable securities	17,924	37,038
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments	0	0	$ 2,900
Equity Securities [Member] | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	17,005	2,005
Unrealized Gains	14,461	5,374
Unrealized (Losses)	0	0
Total fair value of available-for-sale marketable securities	$ 31,466	7,379
Equity Securities [Member] | Restricted [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		15,000
Unrealized Gains		76,439
Unrealized (Losses)		0
Total fair value of available-for-sale marketable securities		$ 91,439