Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 643,938	$ 631,477
Unrealized Gains	73,726	81,989
Unrealized (Losses)	(102)	(1,551)
Total fair value of available-for-sale marketable securities	717,562us-gaap_AvailableForSaleSecurities	711,915us-gaap_AvailableForSaleSecurities
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	234,257	251,761
Maturities after one year through five years	392,624	360,208
Maturities after five years through ten years	1,101	1,128
Total	627,982	613,097
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	140,658	426,941
Fair Value - 12 Months or Greater	4,139	0
Fair Value - Total	144,797	426,941
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(88)	(1,551)
Unrealized Loss - 12 Months or Greater	(14)	0
Unrealized Loss - Total	(102)	(1,551)
U.S. government and government agency obligations
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		25,549
Fair Value - 12 Months or Greater		0
Fair Value - Total		25,549
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months		(46)
Unrealized Loss - 12 Months or Greater		0
Unrealized Loss - Total		(46)
Corporate bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	136,720	390,613
Fair Value - 12 Months or Greater	4,139	0
Fair Value - Total	140,859	390,613
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(85)	(1,462)
Unrealized Loss - 12 Months or Greater	(14)	0
Unrealized Loss - Total	(99)	(1,462)
Municipal bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	3,938	10,779
Fair Value - 12 Months or Greater	0	0
Fair Value - Total	3,938	10,779
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(3)	(43)
Unrealized Loss - 12 Months or Greater	0	0
Unrealized Loss - Total	(3)	(43)
Unrestricted [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	643,938	616,477
Unrealized Gains	73,726	5,550
Unrealized (Losses)	(102)	(1,551)
Total fair value of available-for-sale marketable securities	717,562	620,476
Unrestricted [Member] \| U.S. government and government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	56,433	28,596
Unrealized Gains	201	3
Unrealized (Losses)	0	(46)
Total fair value of available-for-sale marketable securities	56,634	28,553
Unrestricted [Member] \| Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	530,693	548,832
Unrealized Gains	898	136
Unrealized (Losses)	(99)	(1,462)
Total fair value of available-for-sale marketable securities	531,492	547,506
Unrestricted [Member] \| Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	39,807	37,044
Unrealized Gains	52	37
Unrealized (Losses)	(3)	(43)
Total fair value of available-for-sale marketable securities	39,856	37,038
Unrestricted [Member] \| Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	17,005	2,005
Unrealized Gains	72,575	5,374
Unrealized (Losses)	0	0
Total fair value of available-for-sale marketable securities	89,580	7,379
Restricted [Member] \| Equity Securities
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

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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