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Borrowing Arrangements - Summary of Unsecured Credit Facility (Detail)
6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Debt Disclosure [Abstract]    
Weighted average rates 2.96% 3.22%
Range of effective rates, minimum 2.66% 2.69%
Range of effective rates, maximum 3.91% 3.91%