Quarterly report pursuant to Section 13 or 15(d)

Shareholders' Equity - Additional Information (Detail)

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Shareholders' Equity - Additional Information (Detail) - USD ($)
3 Months Ended
Jan. 20, 2016
Mar. 31, 2016
May. 12, 2014
Stockholders Equity [Line Items]      
Common stock shares issued under equity offering program 2,645,000    
Common stock, price per share, public offering $ 105.75    
Proceeds from issuance of common stock $ 269,700,000    
Common stock value authorized under equity offering program     $ 225,000,000
Shares issued under dividend reinvestment plan   44,018  
Equity Program [Member]      
Stockholders Equity [Line Items]      
Common stock, value reserved for future issuance   $ 59,300,000