Marketable Securities (Details) (USD $)	6 Months Ended
	Jun. 30, 2010	Jun. 30, 2011 Integer	Dec. 31, 2010 Integer
Debt Securities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Total		$ 309,768,000	$ 511,625,000
Debt Securities, Fair Value [Abstract]
Fair Value - Total		310,378,000	510,276,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Total		777,000	586,000
Unrealized (Losses) - Total		(167,000)	(1,935,000)
Unrealized Net - Total		610,000	(1,349,000)
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		30,344,000	369,215,000
Fair Value - 12 Months or Greater		207,000	1,128,000
Fair Value - Total		30,551,000	370,343,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 Months		(26,000)	(2,227,000)
Unrealized Loss - 12 Months or Greater		(141,000)	(141,000)
Unrealized Loss - Total		(167,000)	(2,368,000)
Assets, fair value disclosure [Abstract]
Other-than-temporary impairment charge on Level 2 marketable security	100,000
Number of Level 3 marketable securities held		1	1
Unrestricted [Member]
Debt Securities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Total		301,704,000	504,568,000
Debt Securities, Fair Value [Abstract]
Fair Value - Total		302,307,000	503,237,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Total		768,000	586,000
Unrealized (Losses) - Total		(165,000)	(1,917,000)
Unrealized Net - Total		603,000	(1,331,000)
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		28,913,000	363,061,000
Fair Value - 12 Months or Greater		207,000	1,128,000
Fair Value - Total		29,120,000	364,189,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 Months		(24,000)	(2,209,000)
Unrealized Loss - 12 Months or Greater		(141,000)	(141,000)
Unrealized Loss - Total		(165,000)	(2,350,000)
Unrestricted [Member] | Measured on a Recurring Basis [Member]
Assets, fair value disclosure [Abstract]
U.S. government obligations		237,229,000	434,367,000
U.S. government guaranteed corporate bonds		47,496,000	64,008,000
U.S. government guaranteed collateralized mortgage obligations		1,322,000	2,131,000
Municipal bonds		16,053,000	1,603,000
Mortgage-backed securities		207,000	1,128,000
Equity securities		4,320,000	3,612,000
Available-for-sale marketable securities		306,627,000	506,849,000
Unrestricted [Member] | Measured on a Recurring Basis [Member] | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets, fair value disclosure [Abstract]
Equity securities		4,320,000	3,612,000
Available-for-sale marketable securities		4,320,000	3,612,000
Unrestricted [Member] | Measured on a Recurring Basis [Member] | Fair Value Measurements at Reporting Date Using Significant Other Observable Inputs (Level 2) [Member]
Assets, fair value disclosure [Abstract]
U.S. government obligations		237,229,000	434,367,000
U.S. government guaranteed corporate bonds		47,496,000	64,008,000
U.S. government guaranteed collateralized mortgage obligations		1,322,000	2,131,000
Municipal bonds		16,053,000	1,603,000
Mortgage-backed securities		207,000	1,128,000
Available-for-sale marketable securities		302,307,000	503,237,000
Unrestricted [Member] | Equity Securities [Member]
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months			3,612,000
Fair Value - Total			3,612,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 Months			(433,000)
Unrealized Loss - Total			(433,000)
Unrestricted [Member] | U.S. Government Obligations [Member]
Debt Securities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year		9,142,000	83,635,000
Amortized Cost Basis - Maturities between one and five years		227,542,000	352,345,000
Debt Securities, Fair Value [Abstract]
Fair Value - Maturities within one year		9,159,000	83,684,000
Fair Value - Maturities between one and five years		228,070,000	350,683,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities within one year		17,000	54,000
Unrealized Gains - Maturities between one and five years		551,000	64,000
Unrealized (Losses) - Maturities within one year			(5,000)
Unrealized (Losses) - Maturities between one and five years		(23,000)	(1,726,000)
Unrealized Net - Maturities within one year		17,000	49,000
Unrealized Net - Maturities between one and five years		528,000	(1,662,000)
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		25,649,000	340,444,000
Fair Value - Total		25,649,000	340,444,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 Months		(23,000)	(1,731,000)
Unrealized Loss - Total		(23,000)	(1,731,000)
Unrestricted [Member] | U.S. Government Guaranteed Corporate Bonds [Member]
Debt Securities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year		31,936,000	48,173,000
Amortized Cost Basis - Maturities between one and five years		15,393,000	15,522,000
Debt Securities, Fair Value [Abstract]
Fair Value - Maturities within one year		32,088,000	48,531,000
Fair Value - Maturities between one and five years		15,408,000	15,477,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities within one year		152,000	358,000
Unrealized Gains - Maturities between one and five years		15,000
Unrealized (Losses) - Maturities between one and five years			(45,000)
Unrealized Net - Maturities within one year		152,000	358,000
Unrealized Net - Maturities between one and five years		15,000	(45,000)
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months			19,005,000
Fair Value - Total			19,005,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 Months			(45,000)
Unrealized Loss - Total			(45,000)
Unrestricted [Member] | Mortgage-Backed Securities [Member]
Debt Securities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year		73,000	875,000
Amortized Cost Basis - Maturities between one and five years		275,000	110,000
Amortized Cost Basis - Maturities between five and seven years			284,000
Debt Securities, Fair Value [Abstract]
Fair Value - Maturities within one year		72,000	847,000
Fair Value - Maturities between one and five years		135,000	38,000
Fair Value - Maturities between five and seven years			243,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized (Losses) - Maturities within one year		(1,000)	(28,000)
Unrealized (Losses) - Maturities between one and five years		(140,000)	(72,000)
Unrealized (Losses) - Maturities between five and seven years			(41,000)
Unrealized Net - Maturities within one year		(1,000)	(28,000)
Unrealized Net - Maturities between one and five years		(140,000)	(72,000)
Unrealized Net - Maturities between five and seven years			(41,000)
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - 12 Months or Greater		207,000	1,128,000
Fair Value - Total		207,000	1,128,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - 12 Months or Greater		(141,000)	(141,000)
Unrealized Loss - Total		(141,000)	(141,000)
Unrestricted [Member] | U.S. Government Guaranteed Collateralized Mortgage Obligations [Member]
Debt Securities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year		1,321,000	2,027,000
Debt Securities, Fair Value [Abstract]
Fair Value - Maturities within one year		1,322,000	2,131,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities within one year		1,000	104,000
Unrealized Net - Maturities within one year		1,000	104,000
Unrestricted [Member] | Municipal Bonds [Member]
Debt Securities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year		12,712,000	1,597,000
Amortized Cost Basis - Maturities between one and five years		3,310,000
Debt Securities, Fair Value [Abstract]
Fair Value - Maturities within one year		12,730,000	1,603,000
Fair Value - Maturities between one and five years		3,323,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities within one year		19,000	6,000
Unrealized Gains - Maturities between one and five years		13,000
Unrealized (Losses) - Maturities within one year		(1,000)
Unrealized Net - Maturities within one year		18,000	6,000
Unrealized Net - Maturities between one and five years		13,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		3,264,000
Fair Value - Total		3,264,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 Months		(1,000)
Unrealized Loss - Total		(1,000)
Unrestricted [Member] | Debt Securities [Member]
Debt Securities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year		55,184,000	136,307,000
Amortized Cost Basis - Maturities between one and five years		246,520,000	367,977,000
Debt Securities, Fair Value [Abstract]
Fair Value - Maturities within one year		55,371,000	136,796,000
Fair Value - Maturities between one and five years		246,936,000	366,198,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities within one year		189,000	522,000
Unrealized Gains - Maturities between one and five years		579,000	64,000
Unrealized (Losses) - Maturities within one year		(2,000)	(33,000)
Unrealized (Losses) - Maturities between one and five years		(163,000)	(1,843,000)
Unrealized Net - Maturities within one year		187,000	489,000
Unrealized Net - Maturities between one and five years		416,000	(1,779,000)
Restricted [Member]
Debt Securities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Total		8,064,000	7,057,000
Debt Securities, Fair Value [Abstract]
Fair Value - Total		8,071,000	7,039,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Total		9,000
Unrealized (Losses) - Total		(2,000)	(18,000)
Unrealized Net - Total		7,000	(18,000)
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		1,431,000	6,154,000
Fair Value - Total		1,431,000	6,154,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 Months		(2,000)	(18,000)
Unrealized Loss - Total		(2,000)	(18,000)
Restricted [Member] | Measured on a Recurring Basis [Member]
Assets, fair value disclosure [Abstract]
U.S. government obligations		8,071,000	7,039,000
Restricted [Member] | Measured on a Recurring Basis [Member] | Fair Value Measurements at Reporting Date Using Significant Other Observable Inputs (Level 2) [Member]
Assets, fair value disclosure [Abstract]
U.S. government obligations		8,071,000	7,039,000
Restricted [Member] | U.S. Government Obligations [Member]
Debt Securities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year		2,888,000	2,922,000
Amortized Cost Basis - Maturities between one and three years		5,176,000	4,135,000
Debt Securities, Fair Value [Abstract]
Fair Value - Maturities within one year		2,889,000	2,921,000
Fair Value - Maturities between one and three years		5,182,000	4,118,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities within one year		1,000
Unrealized Gains - Maturities between one and three years		8,000
Unrealized (Losses) - Maturities within one year			(1,000)
Unrealized (Losses) - Maturities between one and three years		(2,000)	(17,000)
Unrealized Net - Maturities within one year		1,000	(1,000)
Unrealized Net - Maturities between one and three years		6,000	(17,000)
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		1,431,000	6,154,000
Fair Value - Total		1,431,000	6,154,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 Months		(2,000)	(18,000)
Unrealized Loss - Total		(2,000)	(18,000)
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value of equity securities		4,300,000	3,600,000
Cost basis of equity securities		4,000,000	4,000,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Total		300,000
Unrealized (Losses) - Total			(400,000)
Measured on a Recurring Basis [Member]
Assets, fair value disclosure [Abstract]
Available-for-sale marketable securities		314,698,000	513,888,000
Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets, fair value disclosure [Abstract]
Available-for-sale marketable securities		4,320,000	3,612,000
Fair Value Measurements at Reporting Date Using Significant Other Observable Inputs (Level 2) [Member]
Assets, fair value disclosure [Abstract]
Available-for-sale marketable securities		$ 310,378,000	$ 510,276,000