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Shareholders' Equity - Summary of Changes in Shareholders' Equity (Parenthetical) (Detail) - $ / shares
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Equity [Abstract]        
Dividends per share $ 0.32 $ 0.28 $ 0.64 $ 0.56