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Declared Common Stock Dividends (Detail) - $ / shares
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dividends Payable [Line Items]        
Per Share $ 0.43 $ 0.42 $ 0.86 $ 0.84
Dividend Payment 1st        
Dividends Payable [Line Items]        
Declaration Date     Feb. 17, 2017  
Record Date     Apr. 03, 2017  
Payable Date     Apr. 17, 2017  
Per Share     $ 0.42  
Dividend Payment 2nd        
Dividends Payable [Line Items]        
Declaration Date     May 11, 2017  
Record Date     Jul. 03, 2017  
Payable Date     Jul. 17, 2017  
Per Share     $ 0.42  
Dividend Payment 3rd        
Dividends Payable [Line Items]        
Declaration Date     Aug. 10, 2017  
Record Date     Oct. 02, 2017  
Payable Date     Oct. 16, 2017  
Per Share     $ 0.42  
Dividend Payment 4th        
Dividends Payable [Line Items]        
Declaration Date     Dec. 07, 2017  
Record Date     Jan. 02, 2018  
Payable Date     Jan. 15, 2018  
Per Share     $ 0.42  
Dividend Payment 5th        
Dividends Payable [Line Items]        
Declaration Date     Feb. 22, 2018  
Record Date     Apr. 02, 2018  
Payable Date     Apr. 16, 2018  
Per Share     $ 0.43  
Dividend Payment 6th        
Dividends Payable [Line Items]        
Declaration Date     May 11, 2018  
Record Date     Jul. 02, 2018  
Payable Date     Jul. 16, 2018  
Per Share     $ 0.43