Derivative Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	12 Months Ended			0 Months Ended	12 Months Ended
	Apr. 16, 2010 2006 CMBS Certificate Swaps [Member]	Dec. 31, 2011 2006 CMBS Certificate Swaps [Member]	Dec. 31, 2010 2006 CMBS Certificate Swaps [Member]	Dec. 31, 2009 2006 CMBS Certificate Swaps [Member]	Jul. 28, 2009 2005 CMBS Certificate Swaps [Member]	Dec. 31, 2011 2005 CMBS Certificate Swaps [Member]	Dec. 31, 2009 2005 CMBS Certificate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Deferred gain (loss) on cash flow hedge		$ (12.8)				$ 14.8
Amortization of deferred loss period (years)		five				five
Swap increase in effective interest rate		0.30%				0.80%
Wrote-off remaining unamortized net deferred gain(loss)	3.6				3.9
Amortization of non-cash interest expense			$ 0.6	$ 2.3			$ 1.7
Interest rate swap agreements date						Jun. 22, 2005
Purchase agreement date						Nov. 04, 2005