Fair Values Of Assets And Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money Market Funds, at Carrying Value	$ 1,542,706	$ 1,434,592
Interest Rate Cap Agreements Purchased	2,194	4,548
Interest Rate Swap Agreements - Assets	454	2,004
Derivative Assets	1,545,354	1,441,144
Interest Rate Swap Agreements - Liabilities	456	6,440
Interest Rate Cap Agreements - Sold	2,276	4,768
Derivative Liabilities	2,732	11,208
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money Market Funds, at Carrying Value	1,542,706	1,434,592
Derivative Assets	1,542,706	1,434,592
Derivative Liabilities	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Cap Agreements Purchased	2,194	4,548
Derivative Assets	2,194	4,548
Interest Rate Cap Agreements - Sold	2,276	4,768
Derivative Liabilities	2,276	4,768
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Swap Agreements - Assets	454	2,004
Derivative Assets	454	2,004
Interest Rate Swap Agreements - Liabilities	456	6,440
Derivative Liabilities	$ 456	$ 6,440