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Financing Agreement - Additional Information (Detail) - USD ($)
1 Months Ended
Dec. 31, 2013
Dec. 31, 2010
Sep. 30, 2017
Dec. 31, 2016
Nov. 30, 2016
Sep. 30, 2015
Aug. 31, 2011
Senior Notes | Senior Unsecured Notes 3.81% Due May 2018              
Debt Instrument [Line Items]              
Proceeds from Issuance of Debt   $ 25,000,000          
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          
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            
Mortgage note payable     100,000,000 100,000,000      
Notes payable and other              
Debt Instrument [Line Items]              
Mortgage note payable     12,383,000 13,618,000      
Notes payable and other | Mortgages [Member]              
Debt Instrument [Line Items]              
Mortgage note payable     12,083,000     $ 14,500,000  
Revolving Credit Facility              
Debt Instrument [Line Items]              
Maximum borrowing capacity     600,000,000     $ 500,000,000 $ 350,000,000
Revolving loan facility     0 172,142,000      
Term loan              
Debt Instrument [Line Items]              
Maximum borrowing capacity         $ 750,000,000    
Debt Instrument, Face Amount     500,000,000        
Revolving loan facility     $ 690,000,000 $ 740,000,000