Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		$ 631,477
Unrealized Gains		81,989
Unrealized (Losses)		(1,551)
Total fair value of available-for-sale marketable securities		711,915
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	$ 213,694	251,761
Maturities after one year through five years	542,369	360,208
Maturities after five years through ten years	1,102	1,128
Total	757,165	613,097
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	446,866	426,941
Fair Value - 12 Months or Greater	7,602	0
Fair Value - Total	454,468	426,941
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(954)	(1,551)
Unrealized Loss - 12 Months or Greater	(31)	0
Unrealized Loss - Total	(985)	(1,551)
U.S. government and government agency obligations
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	20,397	25,549
Fair Value - 12 Months or Greater	0	0
Fair Value - Total	20,397	25,549
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(24)	(46)
Unrealized Loss - 12 Months or Greater	0	0
Unrealized Loss - Total	(24)	(46)
Corporate bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	413,794	390,613
Fair Value - 12 Months or Greater	7,602	0
Fair Value - Total	421,396	390,613
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(920)	(1,462)
Unrealized Loss - 12 Months or Greater	(31)	0
Unrealized Loss - Total	(951)	(1,462)
Municipal bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	12,675	10,779
Fair Value - 12 Months or Greater	0	0
Fair Value - Total	12,675	10,779
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(10)	(43)
Unrealized Loss - 12 Months or Greater	0	0
Unrealized Loss - Total	(10)	(43)
Unrestricted [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	774,851	616,477
Unrealized Gains	29,423	5,550
Unrealized (Losses)	(985)	(1,551)
Total fair value of available-for-sale marketable securities	803,289	620,476
Unrestricted [Member] | U.S. government and government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	56,998	28,596
Unrealized Gains	31	3
Unrealized (Losses)	(24)	(46)
Total fair value of available-for-sale marketable securities	57,005	28,553
Unrestricted [Member] | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	661,084	548,832
Unrealized Gains	256	136
Unrealized (Losses)	(951)	(1,462)
Total fair value of available-for-sale marketable securities	660,389	547,506
Unrestricted [Member] | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	39,764	37,044
Unrealized Gains	17	37
Unrealized (Losses)	(10)	(43)
Total fair value of available-for-sale marketable securities	39,771	37,038
Unrestricted [Member] | Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	17,005	2,005
Unrealized Gains	29,119	5,374
Unrealized (Losses)	0	0
Total fair value of available-for-sale marketable securities	$ 46,124	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