Other Assets, Net - Components of Other Assets (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Goodwill And Intangible Assets Disclosure [Abstract]
Amortization expense	$ 6.7	$ 3.2
Straight-line rents receivable, net	$ 59.4		$ 58.2

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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The cumulative difference between the rental payments required by a lease agreement and the rental income or expense recognized on a straight-line basis, or other systematic and rational basis more representative of the time pattern in which use or benefit is granted or derived from the leased property, expected to be recognized in income or expense over the term of the leased property, by the lessor or lessee, respectively. Such receivable is reduced by allowances attributable to, for instance, credit risk associated with a lessee.

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