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Nature of Business and Summary of Significant Accounting Policies - Cash Dividends (Detail) - $ / shares
3 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dividends [Abstract]                        
Dividends per Common Share $ 0.09 $ 0.09 $ 0.09 $ 0.09 $ 0.09 $ 0.09 $ 0.09 $ 0.09 $ 0.09 $ 0.09 $ 0.09 $ 0.09