Derivative Instruments (Schedule of Fair Value Holding Percentage) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Derivative Instruments, Fair Value [Line Items]
Derivative Asset, Fair Value, Net	$ 32.3	$ 29.2
Derivative Instrument Holding Percentage	100.00%	100.00%
Credit Suisse [Member]
Derivative Instruments, Fair Value [Line Items]
Derivative Instrument Holding Percentage	38.00%	40.00%
Wells Fargo [Member]
Derivative Instruments, Fair Value [Line Items]
Derivative Instrument Holding Percentage	13.00%	2.00%
Bnp Paribas [Member]
Derivative Instruments, Fair Value [Line Items]
Derivative Instrument Holding Percentage	13.00%	33.00%
Societe Generale [Member]
Derivative Instruments, Fair Value [Line Items]
Derivative Instrument Holding Percentage	35.00%	22.00%
Bbva Compass [Member]
Derivative Instruments, Fair Value [Line Items]
Derivative Instrument Holding Percentage	1.00%	3.00%