Derivatives and Hedging Activities (Schedule of Interest Rate Derivatives) (Details) (Derivative Financial Instruments, Liabilities [Member], Cash Flow Hedging [Member], Designated as Hedging Instrument [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2013 derivative	Dec. 31, 2012 derivative
Derivative [Line Items]
Number of Interest Rate Derivatives Held	10	8
Notional Amount of Interest Rate Derivatives	$ 717,590	$ 202,590
Interest Rate Derivative Liabilities, at Fair Value	5,012	3,830
Swap [Member]
Derivative [Line Items]
Number of Interest Rate Derivatives Held	9	7
Notional Amount of Interest Rate Derivatives	667,590	152,590
Interest Rate Cap [Member]
Derivative [Line Items]
Number of Interest Rate Derivatives Held	1	1
Notional Amount of Interest Rate Derivatives	50,000	50,000
Interest Rate Derivative Liabilities, at Fair Value	$ 0	$ 0