Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity - Additional Information (Detail)

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Shareholders' Equity - Additional Information (Detail) - USD ($)
12 Months Ended
May 25, 2016
Jan. 20, 2016
Mar. 03, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Aug. 02, 2017
May 31, 2017
Stockholders Equity [Line Items]                
Common stock shares issued under equity offering program 6,900,000 2,645,000 1,380,000          
Common stock, price per share, public offering $ 100.00 $ 105.75 $ 90.40          
Proceeds from issuance of common stock $ 665,400,000 $ 269,700,000 $ 119,500,000          
Common stock value authorized under equity offering program               $ 225,000,000
Offering related costs, commissions and other expenses           $ 200,000    
Buyback Program, authorized repurchase amount             $ 200,000,000  
Stock repurchased during period, shares       112,554        
Stock repurchased during period, value       $ 8,234,000        
Stock repurchased during period, weighted average purchase price       $ 73.16        
Shares issued under dividend reinvestment plan       199,809 133,666 151,246    
Equity Program [Member]                
Stockholders Equity [Line Items]                
Common stock shares issued under equity offering program           949,911    
Proceeds from issuance of common stock           $ 90,600,000    
Weighted average issue price of common stock           $ 96.80    
Equity Program [Member] | Jefferies, Piper and HSBC [Member]                
Stockholders Equity [Line Items]                
Offering related costs, commissions and other expenses           $ 1,100,000