Dividends Declared Per Share (Detail) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Dividends Payable [Line Items]
Dividend Amount	$ 0.365	$ 0.360	$ 1.095	$ 1.080
Group One
Dividends Payable [Line Items]
Declaration Date			Oct. 24, 2018
Date Paid (Payable)			Nov. 29, 2018
Shareholder of Record Date			Nov. 15, 2018
Dividend Amount			$ 0.365
Group Two
Dividends Payable [Line Items]
Declaration Date			Jul. 25, 2018
Date Paid (Payable)			Aug. 29, 2018
Shareholder of Record Date			Aug. 15, 2018
Dividend Amount			$ 0.365
Group Three
Dividends Payable [Line Items]
Declaration Date			Apr. 25, 2018
Date Paid (Payable)			May 29, 2018
Shareholder of Record Date			May 15, 2018
Dividend Amount			$ 0.365
Group Four
Dividends Payable [Line Items]
Declaration Date			Jan. 30, 2018
Date Paid (Payable)			Feb. 28, 2018
Shareholder of Record Date			Feb. 14, 2018
Dividend Amount			$ 0.365
Group Five
Dividends Payable [Line Items]
Declaration Date			Oct. 25, 2017
Date Paid (Payable)			Nov. 29, 2017
Shareholder of Record Date			Nov. 15, 2017
Dividend Amount			$ 0.360
Group Six
Dividends Payable [Line Items]
Declaration Date			Jul. 26, 2017
Date Paid (Payable)			Aug. 29, 2017
Shareholder of Record Date			Aug. 15, 2017
Dividend Amount			$ 0.360
Group Seven
Dividends Payable [Line Items]
Declaration Date			Apr. 26, 2017
Date Paid (Payable)			May 30, 2017
Shareholder of Record Date			May 16, 2017
Dividend Amount			$ 0.360
Group Eight
Dividends Payable [Line Items]
Declaration Date			Jan. 25, 2017
Date Paid (Payable)			Feb. 28, 2017
Shareholder of Record Date			Feb. 14, 2017
Dividend Amount			$ 0.360