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Intangibles (Details) (Leases, Acquired-in-Place [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 Y	Mar. 31, 2011	Dec. 31, 2011
Leases, Acquired-in-Place [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Amortization period (in years)	7
Gross carrying amount	$ 23.4		$ 25.3
Accumulated amortization	10.4		10.8
Aggregate net amortization net expense	$ 1.0	$ 0.5

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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