Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 483,142	$ 354,990
Unrealized Gains	586	670
Unrealized (Losses)	(1,415)	(702)
Available-for-sale Securities, Fair Value Disclosure	482,313	354,958
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total	479,528	351,565
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	146,953	63,442
Fair Value - 12 Months or Greater	2,786	3,393
Fair Value - Total	149,739	66,835
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(156)	(51)
Unrealized Loss - 12 Months or Greater	(1,259)	(651)
Unrealized Loss - Total	(1,415)	(702)
Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	474,792	349,088
Unrealized Gains	577	661
Unrealized (Losses)	(1,414)	(700)
Available-for-sale Securities, Fair Value Disclosure	473,955	349,049
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	127,349	77,819
Maturities after one year through five years	343,821	267,837
Total	471,170	345,656
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	146,049	62,248
Fair Value - 12 Months or Greater	2,786	3,393
Fair Value - Total	148,835	65,641
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(155)	(49)
Unrealized Loss - 12 Months or Greater	(1,259)	(651)
Unrealized Loss - Total	(1,414)	(700)
Restricted
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	6,868	2,781
Maturities after one year through five years	1,490	3,128
Total	8,358	5,909
U.S. government and government agency obligations | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	305,402	327,502
Unrealized Gains	568	661
Unrealized (Losses)	(3)	(17)
Available-for-sale Securities, Fair Value Disclosure	305,967	328,146
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	25,004	44,738
Fair Value - Total	25,004	44,738
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(3)	(17)
Unrealized Loss - Total	(3)	(17)
Corporate bond | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	123,015
Unrealized Gains	3
Unrealized (Losses)	(148)
Available-for-sale Securities, Fair Value Disclosure	122,870
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	114,555
Fair Value - Total	114,555
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(148)
Unrealized Loss - Total	(148)
Commercial paper | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	25,172
Unrealized Gains
Unrealized (Losses)
Available-for-sale Securities, Fair Value Disclosure	25,172
Municipal bonds | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	17,159	17,542
Unrealized Gains	6
Unrealized (Losses)	(4)	(32)
Available-for-sale Securities, Fair Value Disclosure	17,161	17,510
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	6,490	17,510
Fair Value - Total	6,490	17,510
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(4)	(32)
Unrealized Loss - Total	(4)	(32)
Equity securities | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	4,044	4,044
Unrealized Gains
Unrealized (Losses)	(1,259)	(651)
Available-for-sale Securities, Fair Value Disclosure	2,785	3,393
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months
Fair Value - 12 Months or Greater	2,786	3,393
Fair Value - Total	2,786	3,393
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months
Unrealized Loss - 12 Months or Greater	(1,259)	(651)
Unrealized Loss - Total	(1,259)	(651)
U.S. government obligations | Restricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	8,350	5,902
Unrealized Gains	9	9
Unrealized (Losses)	(1)	(2)
Available-for-sale Securities, Fair Value Disclosure	8,358	5,909
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	904	1,194
Fair Value - Total	904	1,194
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(1)	(2)
Unrealized Loss - Total	$ (1)	$ (2)