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Marketable Securities (Details) (USD $)
9 Months Ended
Sep. 30, 2010
Sep. 30, 2011
Integer
Dec. 31, 2010
Integer
Debt Securities, Amortized Cost Basis [Abstract]   
Amortized Cost Basis - Total $ 300,861,000$ 511,625,000
Debt Securities, Fair Value [Abstract]   
Fair Value - Total 301,328,000510,276,000
Unrealized Gains (Losses), Net [Abstract]   
Unrealized Gains - Total 673,000586,000
Unrealized (Losses) - Total (206,000)(1,935,000)
Unrealized Net - Total 467,000(1,349,000)
Continuous Unrealized Loss Position, Fair Value [Abstract]   
Fair Value - Less than 12 Months 70,309,000369,215,000
Fair Value - 12 Months or Greater 149,0001,128,000
Fair Value - Total 70,458,000370,343,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]   
Unrealized Loss - Less than 12 Months (1,028,000)(2,227,000)
Unrealized Loss - 12 Months or Greater (147,000)(141,000)
Unrealized Loss - Total (1,175,000)(2,368,000)
Assets, fair value disclosure [Abstract]   
Other-than-temporary impairment charge on Level 2 marketable security100,000  
Number of Level 3 marketable securities held 11
Unrestricted [Member]
   
Continuous Unrealized Loss Position, Fair Value [Abstract]   
Fair Value - Less than 12 Months 70,309,000363,061,000
Fair Value - 12 Months or Greater 149,0001,128,000
Fair Value - Total 70,458,000364,189,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]   
Unrealized Loss - Less than 12 Months (1,028,000)(2,209,000)
Unrealized Loss - 12 Months or Greater (147,000)(141,000)
Unrealized Loss - Total (1,175,000)(2,350,000)
Unrestricted [Member] | Measured on a Recurring Basis [Member]
   
Assets, fair value disclosure [Abstract]   
U.S. government obligations 241,451,000434,367,000
U.S. government guaranteed corporate bonds 36,766,00064,008,000
U.S. government guaranteed collateralized mortgage obligations 986,0002,131,000
Municipal bonds 14,749,0001,603,000
Mortgage-backed securities 149,0001,128,000
Equity securities 3,075,0003,612,000
Available-for-sale marketable securities 297,176,000506,849,000
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 3,075,0003,612,000
Available-for-sale marketable securities 3,075,0003,612,000
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 241,451,000434,367,000
U.S. government guaranteed corporate bonds 36,766,00064,008,000
U.S. government guaranteed collateralized mortgage obligations 986,0002,131,000
Municipal bonds 14,749,0001,603,000
Mortgage-backed securities 149,0001,128,000
Available-for-sale marketable securities 294,101,000503,237,000
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 2,028,00083,635,000
Amortized Cost Basis - Maturities between one and five years 238,958,000352,345,000
Debt Securities, Fair Value [Abstract]   
Fair Value - Maturities within one year 2,033,00083,684,000
Fair Value - Maturities between one and five years 239,418,000350,683,000
Unrealized Gains (Losses), Net [Abstract]   
Unrealized Gains - Maturities within one year 5,00054,000
Unrealized Gains - Maturities between one and five years 519,00064,000
Unrealized (Losses) - Maturities within one year  (5,000)
Unrealized (Losses) - Maturities between one and five years (59,000)(1,726,000)
Unrealized Net - Maturities within one year 5,00049,000
Unrealized Net - Maturities between one and five years 460,000(1,662,000)
Continuous Unrealized Loss Position, Fair Value [Abstract]   
Fair Value - Less than 12 Months 67,234,000340,444,000
Fair Value - Total 67,234,000340,444,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]   
Unrealized Loss - Less than 12 Months (59,000)(1,731,000)
Unrealized Loss - Total (59,000)(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 21,249,00048,173,000
Amortized Cost Basis - Maturities between one and five years 15,415,00015,522,000
Debt Securities, Fair Value [Abstract]   
Fair Value - Maturities within one year 21,309,00048,531,000
Fair Value - Maturities between one and five years 15,457,00015,477,000
Unrealized Gains (Losses), Net [Abstract]   
Unrealized Gains - Maturities within one year 60,000358,000
Unrealized Gains - Maturities between one and five years 42,000 
Unrealized (Losses) - Maturities between one and five years  (45,000)
Unrealized Net - Maturities within one year 60,000358,000
Unrealized Net - Maturities between one and five years 42,000(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 26,000875,000
Amortized Cost Basis - Maturities between one and five years  110,000
Amortized Cost Basis - Maturities between five and six years 270,000 
Amortized Cost Basis - Maturities between five and seven years  284,000
Debt Securities, Fair Value [Abstract]   
Fair Value - Maturities within one year 26,000847,000
Fair Value - Maturities between one and five years  38,000
Fair Value - Maturities between five and six years 123,000 
Fair Value - Maturities between five and seven years  243,000
Unrealized Gains (Losses), Net [Abstract]   
Unrealized (Losses) - Maturities within one year  (28,000)
Unrealized (Losses) - Maturities between one and five years  (72,000)
Unrealized (Losses) - Maturities between five and six years (147,000) 
Unrealized (Losses) - Maturities between five and seven years  (41,000)
Unrealized Net - Maturities within one year  (28,000)
Unrealized Net - Maturities between one and five years  (72,000)
Unrealized Net - Maturities between five and six years (147,000) 
Unrealized Net - Maturities between five and seven years  (41,000)
Continuous Unrealized Loss Position, Fair Value [Abstract]   
Fair Value - 12 Months or Greater 149,0001,128,000
Fair Value - Total 149,0001,128,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]   
Unrealized Loss - 12 Months or Greater (147,000)(141,000)
Unrealized Loss - Total (147,000)(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 986,0002,027,000
Debt Securities, Fair Value [Abstract]   
Fair Value - Maturities within one year 986,0002,131,000
Unrealized Gains (Losses), Net [Abstract]   
Unrealized Gains - Maturities within one year  104,000
Unrealized Net - Maturities within one year  104,000
Unrestricted [Member] | Municipal Bonds [Member]
   
Debt Securities, Amortized Cost Basis [Abstract]   
Amortized Cost Basis - Maturities within one year 14,722,0001,597,000
Debt Securities, Fair Value [Abstract]   
Fair Value - Maturities within one year 14,749,0001,603,000
Unrealized Gains (Losses), Net [Abstract]   
Unrealized Gains - Maturities within one year 27,0006,000
Unrealized Net - Maturities within one year 27,0006,000
Unrestricted [Member] | Debt Securities [Member]
   
Debt Securities, Amortized Cost Basis [Abstract]   
Amortized Cost Basis - Maturities within one year 39,011,000136,307,000
Amortized Cost Basis - Maturities between one and five years 254,373,000367,977,000
Amortized Cost Basis - Maturities between five and six years 270,000 
Amortized Cost Basis - Maturities between five and seven years  284,000
Amortized Cost Basis - Total 293,654,000504,568,000
Debt Securities, Fair Value [Abstract]   
Fair Value - Maturities within one year 39,103,000136,796,000
Fair Value - Maturities between one and five years 254,875,000366,198,000
Fair Value - Maturities between five and six years 123,000 
Fair Value - Maturities between five and seven years  243,000
Fair Value - Total 294,101,000503,237,000
Unrealized Gains (Losses), Net [Abstract]   
Unrealized Gains - Maturities within one year 92,000522,000
Unrealized Gains - Maturities between one and five years 561,00064,000
Unrealized Gains - Total 653,000586,000
Unrealized (Losses) - Maturities within one year  (33,000)
Unrealized (Losses) - Maturities between one and five years (59,000)(1,843,000)
Unrealized (Losses) - Maturities between five and six years (147,000) 
Unrealized (Losses) - Maturities between five and seven years  (41,000)
Unrealized (Losses) - Total (206,000)(1,917,000)
Unrealized Net - Maturities within one year 92,000489,000
Unrealized Net - Maturities between one and five years 502,000(1,779,000)
Unrealized Net - Maturities between five and six years (147,000) 
Unrealized Net - Maturities between five and seven years  (41,000)
Unrealized Net - Total 447,000(1,331,000)
Restricted [Member]
   
Continuous Unrealized Loss Position, Fair Value [Abstract]   
Fair Value - Less than 12 Months  6,154,000
Fair Value - Total  6,154,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]   
Unrealized Loss - Less than 12 Months  (18,000)
Unrealized Loss - Total  (18,000)
Restricted [Member] | Measured on a Recurring Basis [Member]
   
Assets, fair value disclosure [Abstract]   
U.S. government obligations 7,227,0007,039,000
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 7,227,0007,039,000
Restricted [Member] | U.S. Government Obligations [Member]
   
Debt Securities, Amortized Cost Basis [Abstract]   
Amortized Cost Basis - Maturities within one year 2,946,0002,922,000
Amortized Cost Basis - Maturities between one and three years 4,261,0004,135,000
Debt Securities, Fair Value [Abstract]   
Fair Value - Maturities within one year 2,947,0002,921,000
Fair Value - Maturities between one and three years 4,280,0004,118,000
Unrealized Gains (Losses), Net [Abstract]   
Unrealized Gains - Maturities within one year 1,000 
Unrealized Gains - Maturities between one and three years 19,000 
Unrealized (Losses) - Maturities within one year  (1,000)
Unrealized (Losses) - Maturities between one and three years  (17,000)
Unrealized Net - Maturities within one year 1,000(1,000)
Unrealized Net - Maturities between one and three years 19,000(17,000)
Continuous Unrealized Loss Position, Fair Value [Abstract]   
Fair Value - Less than 12 Months  6,154,000
Fair Value - Total  6,154,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]   
Unrealized Loss - Less than 12 Months  (18,000)
Unrealized Loss - Total  (18,000)
Restricted [Member] | Debt Securities [Member]
   
Debt Securities, Amortized Cost Basis [Abstract]   
Amortized Cost Basis - Total 7,207,0007,057,000
Debt Securities, Fair Value [Abstract]   
Fair Value - Total 7,227,0007,039,000
Unrealized Gains (Losses), Net [Abstract]   
Unrealized Gains - Total 20,000 
Unrealized (Losses) - Total  (18,000)
Unrealized Net - Total 20,000(18,000)
Equity Securities [Member]
   
Schedule of Available-for-sale Securities [Line Items]   
Fair value of equity securities 3,100,0003,600,000
Cost basis of equity securities 4,000,0004,000,000
Unrealized Gains (Losses), Net [Abstract]   
Unrealized (Losses) - Total (900,000)(400,000)
Continuous Unrealized Loss Position, Fair Value [Abstract]   
Fair Value - Less than 12 Months 3,075,000 
Fair Value - Total 3,075,000 
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]   
Unrealized Loss - Less than 12 Months (969,000) 
Unrealized Loss - Total (969,000) 
Measured on a Recurring Basis [Member]
   
Assets, fair value disclosure [Abstract]   
Available-for-sale marketable securities 304,403,000513,888,000
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 3,075,0003,612,000
Fair Value Measurements at Reporting Date Using Significant Other Observable Inputs (Level 2) [Member]
   
Assets, fair value disclosure [Abstract]   
Available-for-sale marketable securities $ 301,328,000$ 510,276,000