Dividends Declared Per Share (Detail) - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Dividends Payable [Line Items]
Dividend Amount	$ 0.365	$ 0.360
Group One
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 Two
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 Three
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 Four
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 Five
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 Six
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