Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Fair Value Measurements
Cash equivalents	$ 1,993,951	$ 2,490,809
Interest rate caps	1,316	1,617
Fair Value Measurements (Textuals) [Abstract]
Number of interest rate cap agreements	38	34
Fair value of interest rate cap agreements	1,316	1,617
Quoted Market Prices in Active Markets (Level 1) [Member]
Fair Value Measurements
Cash equivalents	1,993,951	2,490,809
Interest rate caps	0	0
Fair Value Measurements (Textuals) [Abstract]
Fair value of interest rate cap agreements	0	0
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Measurements
Cash equivalents	0	0
Interest rate caps	1,316	1,617
Fair Value Measurements (Textuals) [Abstract]
Fair value of interest rate cap agreements	1,316	1,617
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements
Cash equivalents	0	0
Interest rate caps	0	0
Fair Value Measurements (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.

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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 derivative financial instruments which the Company is a party to as of the balance sheet date. A derivative instrument is a financial instrument or other contract with all three of the following characteristics: (a) it has (1) one or more underlyings and (2) one or more notional amounts or payment provisions or both. Those terms determine the amount of the settlement or settlements, and, in some cases, whether or not a settlement is required; (b) it requires no initial net investment or an initial net investment that is smaller than would be required for other types of contracts that would be expected to have a similar response to changes in market factors; (c) its terms require or permit net settlement, it can readily be settled net by a means outside the contract, or it provides for delivery of an asset that puts the recipient in a position not substantially different from net settlement. Notwithstanding the forgoing characteristics, loan commitments that relate to the origination of mortgage loans that will be held for sale shall be accounted for as derivative instruments by the issuer of the loan commitment (that is, the potential lender).

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