Real estate (Details 6) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Real Estate [Abstract]
Amortization expense related to real estate intangibles	$ 10,600,000	$ 11,000,000	$ 9,000,000
Unamortized finite lived intangible assets	59,900,000	53,600,000
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2013	11,534,000
2014	9,221,000
2015	7,589,000
2016	6,770,000
2017	$ 5,721,000

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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