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Subsequent Event - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Billions
12 Months Ended
Feb. 15, 2018
Jan. 11, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Subsequent Event [Line Items]          
Cash dividends declared per share     $ 0.265 $ 0.25 $ 0.25
Subsequent Event [Member]          
Subsequent Event [Line Items]          
Cash dividends declared per share $ 0.07        
Dividend declared date Feb. 15, 2018        
Dividend paid date Mar. 15, 2018        
Dividend record date Mar. 01, 2018        
Subsequent Event [Member] | DST [Member]          
Subsequent Event [Line Items]          
Business acquisition, effective date of acquisition   Jan. 11, 2018      
Enterprise value of acquisition   $ 5.4      
Business acquisition, share Price   $ 84