Quarterly report pursuant to Section 13 or 15(d)

Shareholders' Equity - Additional Information (Detail)

v3.10.0.1
Shareholders' Equity - Additional Information (Detail) - USD ($)
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Jun. 14, 2018
Aug. 02, 2017
Stockholders Equity [Line Items]        
Common stock value authorized under equity offering program     $ 300,000,000  
Buyback Program, authorized repurchase amount       $ 200,000,000
Stock repurchased during period, shares 0      
Shares issued under dividend reinvestment plan   199,809    
Equity Program [Member]        
Stockholders Equity [Line Items]        
Common stock remains available for issuance $ 300,000,000