Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
Sep. 30, 2011 warrants derivative	Dec. 31, 2010 derivative
Fair Value Measurements
Interest rate caps	$ 1,418	$ 1,617
Financial Instruments, Financial Assets, Balance Sheet Groupings (Textuals) [Abstract]
Maturity period of cash equivalents	Less than 90 days
Number of interest rate cap agreements	39	34
Fair value of interest rate cap agreements	1,418	1,617
Fair Value, Measurements, Recurring [Member]
Fair Value Measurements
Cash equivalents	2,229,081	2,490,809
Interest rate caps	1,418	1,617
Financial Instruments, Financial Assets, Balance Sheet Groupings (Textuals) [Abstract]
Fair value of interest rate cap agreements	1,418	1,617
Fair Value, Measurements, Recurring [Member] \| Quoted Market Prices in Active Markets (Level 1) [Member]
Fair Value Measurements
Cash equivalents	2,229,081	2,490,809
Interest rate caps	0	0
Financial Instruments, Financial Assets, Balance Sheet Groupings (Textuals) [Abstract]
Fair value of interest rate cap agreements	0	0
Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value Measurements
Cash equivalents	0	0
Interest rate caps	1,418	1,617
Financial Instruments, Financial Assets, Balance Sheet Groupings (Textuals) [Abstract]
Fair value of interest rate cap agreements	1,418	1,617
Fair Value, Measurements, Recurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements
Cash equivalents	0	0
Interest rate caps	0	0
Financial Instruments, Financial Assets, Balance Sheet Groupings (Textuals) [Abstract]
Fair value of interest rate cap agreements	$ 0	$ 0

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.

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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