Fair Value (Details) (USD $)	6 Months Ended				6 Months Ended
	Jun. 30, 2012 Cash Flow Hedges derivative	Jun. 30, 2011 Cash Flow Hedges	Jun. 30, 2012 Cash Flow Hedges Interest Rate Caps	Jun. 30, 2012 Cash Flow Hedges Interest Rate Swaps	Jun. 30, 2011 Fair Value Hedges	Dec. 31, 2011 Fair Value Hedges derivative	Jun. 30, 2012 Non-Designated Hedges derivative	Jun. 30, 2012 Non-Designated Hedges Interest Rate Caps
Derivative instruments and Hedging Activities
Notional balance			$ 180,195,000	$ 430,000,000				$ 42,747,000
Weighted average interest rate (as a percent)			2.40%	4.50%				1.20%
Weighted average capped interest rate (as a percent)			5.30%					7.30%
Number of derivative instruments held	6					2	3
Amount of increase (decrease) in comprehensive income to adjust hedging derivatives	$ (16,789,000)	$ (19,420,000)			$ 863,000