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Financing Agreement - Additional Information (Detail)
ft² in Thousands
1 Months Ended
Dec. 31, 2013
USD ($)
Dec. 31, 2010
USD ($)
Jun. 30, 2015
USD ($)
Mar. 31, 2015
USD ($)
Jan. 09, 2015
ft²
Dec. 31, 2014
USD ($)
Aug. 31, 2011
USD ($)
Debt Instrument [Line Items]              
Area of Real Estate Property | ft²         600    
Senior Notes | Senior Unsecured Notes 3.81% Due May 2018              
Debt Instrument [Line Items]              
Proceeds from Issuance of Debt   $ 25,000,000          
Debt Instruments Maturity Date   5/31/2018          
Mortgage note payable     $ 25,000,000     $ 25,000,000  
Senior Notes | Senior Unsecured Notes 4.60% Due May 2021              
Debt Instrument [Line Items]              
Proceeds from Issuance of Debt   $ 75,000,000          
Debt Instruments Maturity Date   5/31/2021          
Mortgage note payable     75,000,000     75,000,000  
Senior Notes | Senior Unsecured Notes 3.13% Due December 2020              
Debt Instrument [Line Items]              
Proceeds from Issuance of Debt $ 100,000,000            
Debt Instruments Maturity Date 12/31/2020            
Mortgage note payable     100,000,000     100,000,000  
Notes payable              
Debt Instrument [Line Items]              
Mortgage note payable     $ 15,943,000     $ 0  
Notes payable | Mortgages [Member]              
Debt Instrument [Line Items]              
Mortgage note payable       $ 14,500,000      
Revolving Credit Facility              
Debt Instrument [Line Items]              
Maximum borrowing capacity       $ 500,000,000     $ 350,000,000