Marketable Securities (Details) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Debt Maturities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Total	$ 460,911,000	$ 321,845,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Total	812,000	529,000
Unrealized (Losses) - Total	(84,000)	(236,000)
Debt Maturities, Fair Value [Abstract]
Fair Value - Total	461,639,000	322,138,000
Unrestricted
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	63,517,000	111,774,000
Fair Value - 12 Months or Greater	2,634,000	116,000
Fair Value - Total	66,151,000	111,890,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(18,000)	(1,108,000)
Unrealized Loss - 12 Months or Greater	(1,567,000)	(152,000)
Unrealized Loss - Total	(1,585,000)	(1,260,000)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Fair value of equity securities	2,500,000	3,000,000
Cost basis of equity securities	4,000,000	4,000,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized (Losses) - Total	(1,500,000)	(1,000,000)
Equity securities | Unrestricted
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		3,019,000
Fair Value - 12 Months or Greater	2,543,000
Fair Value - Total	2,543,000	3,019,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months		(1,025,000)
Unrealized Loss - 12 Months or Greater	(1,501,000)
Unrealized Loss - Total	(1,501,000)	(1,025,000)
Debt securities | Unrestricted
Debt Maturities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year	143,458,000	43,225,000
Amortized Cost Basis - Maturities after one year through five years	309,281,000	272,537,000
Amortized Cost Basis - Maturities after five years through ten years	157,000	164,000
Amortized Cost Basis - Total	452,896,000	315,926,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities within one year	102,000	108,000
Unrealized Gains - Maturities after one year through five years	697,000	400,000
Unrealized Gains - Total	799,000	508,000
Unrealized (Losses) - Maturities within one year	(2,000)	(2,000)
Unrealized (Losses) - Maturities after one year through five years	(16,000)	(157,000)
Unrealized (Losses) - Maturities after five years through ten years	(66,000)	(77,000)
Unrealized (Losses) - Total	(84,000)	(236,000)
Debt Maturities, Fair Value [Abstract]
Fair Value - Maturities within one year	143,558,000	43,331,000
Fair Value - Maturities after one year through five years	309,962,000	272,780,000
Fair Value - Maturities after five years through ten years	91,000	87,000
Fair Value - Total	453,611,000	316,198,000
Debt securities | Restricted
Debt Maturities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Total	8,015,000	5,919,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Total	13,000	21,000
Unrealized (Losses) - Total
Debt Maturities, Fair Value [Abstract]
Fair Value - Total	8,028,000	5,940,000
U.S. government obligations | Unrestricted
Debt Maturities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year	76,443,000	12,025,000
Amortized Cost Basis - Maturities after one year through five years	291,751,000	272,433,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities within one year	84,000	42,000
Unrealized Gains - Maturities after one year through five years	695,000	400,000
Unrealized (Losses) - Maturities within one year	(1,000)
Unrealized (Losses) - Maturities after one year through five years	(9,000)	(81,000)
Debt Maturities, Fair Value [Abstract]
Fair Value - Maturities within one year	76,526,000	12,067,000
Fair Value - Maturities after one year through five years	292,437,000	272,752,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	27,033,000	103,529,000
Fair Value - 12 Months or Greater
Fair Value - Total	27,033,000	103,529,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(10,000)	(81,000)
Unrealized Loss - 12 Months or Greater
Unrealized Loss - Total	(10,000)	(81,000)
U.S. government obligations | Restricted
Debt Maturities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year	4,248,000	3,347,000
Amortized Cost Basis - Maturities after one year through five years	3,767,000	2,572,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities within one year	4,000	10,000
Unrealized Gains - Maturities after one year through five years	9,000	11,000
Unrealized (Losses) - Maturities within one year
Unrealized (Losses) - Maturities after one year through five years
Debt Maturities, Fair Value [Abstract]
Fair Value - Maturities within one year	4,252,000	3,357,000
Fair Value - Maturities after one year through five years	3,776,000	2,583,000
U.S. government guaranteed corporate bonds | Unrestricted
Debt Maturities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year	57,963,000	15,263,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities within one year	18,000	53,000
Unrealized (Losses) - Maturities within one year	(1,000)
Debt Maturities, Fair Value [Abstract]
Fair Value - Maturities within one year	57,980,000	15,316,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	25,411,000
Fair Value - 12 Months or Greater
Fair Value - Total	25,411,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(1,000)
Unrealized Loss - 12 Months or Greater
Unrealized Loss - Total	(1,000)
Municipal bonds | Unrestricted
Debt Maturities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year	9,052,000	15,314,000
Amortized Cost Basis - Maturities after one year through five years	17,530,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities within one year		13,000
Unrealized Gains - Maturities after one year through five years	2,000
Unrealized (Losses) - Maturities within one year		(1,000)
Unrealized (Losses) - Maturities after one year through five years	(7,000)
Debt Maturities, Fair Value [Abstract]
Fair Value - Maturities within one year	9,052,000	15,326,000
Fair Value - Maturities after one year through five years	17,525,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	11,073,000	4,603,000
Fair Value - 12 Months or Greater
Fair Value - Total	11,073,000	4,603,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(7,000)	(1,000)
Unrealized Loss - 12 Months or Greater
Unrealized Loss - Total	(7,000)	(1,000)
Mortgage-backed securities | Unrestricted
Debt Maturities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities after one year through five years		104,000
Amortized Cost Basis - Maturities after five years through ten years	157,000	164,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities after one year through five years
Unrealized Gains - Maturities after five years through ten years
Unrealized (Losses) - Maturities after one year through five years		(76,000)
Unrealized (Losses) - Maturities after five years through ten years	(66,000)	(77,000)
Debt Maturities, Fair Value [Abstract]
Fair Value - Maturities after one year through five years		28,000
Fair Value - Maturities after five years through ten years	91,000	87,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months
Fair Value - 12 Months or Greater	91,000	116,000
Fair Value - Total	91,000	116,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months
Unrealized Loss - 12 Months or Greater	(66,000)	(152,000)
Unrealized Loss - Total	(66,000)	(152,000)
U.S. government guaranteed collateralized mortgage obligations | Unrestricted
Debt Maturities, Amortized Cost Basis [Abstract]
Amortized Cost Basis - Maturities within one year		623,000
Unrealized Gains (Losses), Net [Abstract]
Unrealized Gains - Maturities within one year
Unrealized (Losses) - Maturities within one year		(1,000)
Debt Maturities, Fair Value [Abstract]
Fair Value - Maturities within one year		622,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		623,000
Fair Value - 12 Months or Greater
Fair Value - Total		623,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months		(1,000)
Unrealized Loss - 12 Months or Greater
Unrealized Loss - Total		$ (1,000)