Summary of Significant Accounting Policies (Details 9) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Restricted Cash	$ 236,696,000	$ 226,554,000	$ 185,794,000
Investments	208,313,000	233,690,000
Cash held in bank accounts by the Company, unrestricted	102,200,000	87,300,000
Cash held in bank accounts by the Company, restricted	108,400,000	143,700,000
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	1,129,000	1,065,000
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	436,530,000	417,601,000
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	437,659,000	418,666,000
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	1,129,000	1,065,000
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	101,028,000	102,137,000
Restricted Cash	128,318,000	82,839,000
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	8,440,000	6,128,000
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	198,594,000	214,547,000
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 | 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	150,000
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	101,028,000	102,137,000
Restricted Cash	128,318,000	82,839,000
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	1,129,000	1,065,000
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	8,440,000	6,128,000
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	198,594,000	214,547,000
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
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	$ 150,000