Summary of Significant Accounting Policies (Details 9) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Restricted Cash	$ 226,554,000	$ 185,794,000	$ 116,734,000
Investments	233,690,000	200,903,000
Cash held in bank accounts by the Company, unrestricted	87,300,000	118,600,000
Cash held in bank accounts by the Company, restricted	143,700,000	137,800,000
Level 1
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Total assets measured at fair value on a recurring basis	1,065,000	697,000
Level 2
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Total assets measured at fair value on a recurring basis	417,601,000	249,474,000
Total
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Total assets measured at fair value on a recurring basis	418,666,000	250,171,000
Fair value measured on recurring basis | Level 1 | U.S. Government and agency securities
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Investments	1,065,000	697,000
Fair value measured on recurring basis | Level 2 | Other than cash held in bank accounts by Company
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Cash and Cash Equivalents	102,137,000	1,296,000
Restricted Cash	82,839,000	47,972,000
Fair value measured on recurring basis | Level 2 | Obligations of government-sponsored enterprises
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Investments	6,128,000	8,293,000
Fair value measured on recurring basis | Level 2 | Corporate debt securities
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Investments	214,547,000	191,813,000
Fair value measured on recurring basis | Level 2 | Certificates of deposit
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Investments	150,000	100,000
Fair value measured on recurring basis | Level 2 | Taxable municipal bonds
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Investments	11,800,000
Fair value measured on recurring basis | Total | Other than cash held in bank accounts by Company
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Cash and Cash Equivalents	102,137,000	1,296,000
Restricted Cash	82,839,000	47,972,000
Fair value measured on recurring basis | Total | U.S. Government and agency securities
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Investments	1,065,000	697,000
Fair value measured on recurring basis | Total | Obligations of government-sponsored enterprises
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Investments	6,128,000	8,293,000
Fair value measured on recurring basis | Total | Corporate debt securities
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Investments	214,547,000	191,813,000
Fair value measured on recurring basis | Total | Certificates of deposit
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Investments	150,000	100,000
Fair value measured on recurring basis | Total | Taxable municipal bonds
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Investments	$ 11,800,000