General (Details 4) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Fair value of financial assets and liabilities required to be measured at fair value on a recurring basis
Restricted cash	$ 221,790,000	$ 226,554,000
Investments	250,002,000	233,690,000
Cash held in bank accounts by the Company, unrestricted	116,100,000	87,300,000
Cash held in bank accounts by the Company, restricted	104,700,000	143,700,000
Fair value measured on recurring basis \| 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	1,065,000
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	1,065,000
Fair value measured on recurring basis \| 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	438,339,000	417,601,000
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	72,282,000	102,137,000
Restricted cash	117,120,000	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	8,449,000	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	236,472,000	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	3,866,000	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
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	439,404,000	418,666,000
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	72,282,000	102,137,000
Restricted cash	117,120,000	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	1,065,000	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	8,449,000	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	236,472,000	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	3,866,000	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

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation.

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

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- Definition

The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. For a classified balance sheet represents the current portion only (the noncurrent portion has a separate concept); there is a separate and distinct element for unclassified presentations.

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