Dividends Declared Per Share (Detail) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Dividends Payable [Line Items]
Dividend Amount	$ 0.360	$ 0.355	$ 0.720	$ 0.710
Group Seven
Dividends Payable [Line Items]
Declaration Date			Jul. 25, 2017
Date Paid (Payable)			Aug. 29, 2017
Shareholder of Record Date			Aug. 15, 2017
Dividend Amount			$ 0.360
Group One
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 Two
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
Group Three
Dividends Payable [Line Items]
Declaration Date			Oct. 19, 2016
Date Paid (Payable)			Nov. 28, 2016
Shareholder of Record Date			Nov. 14, 2016
Dividend Amount			$ 0.355
Group Four
Dividends Payable [Line Items]
Declaration Date			Jul. 20, 2016
Date Paid (Payable)			Aug. 26, 2016
Shareholder of Record Date			Aug. 12, 2016
Dividend Amount			$ 0.355
Group Five
Dividends Payable [Line Items]
Declaration Date			Apr. 20, 2016
Date Paid (Payable)			May 27, 2016
Shareholder of Record Date			May 13, 2016
Dividend Amount			$ 0.355
Group Six
Dividends Payable [Line Items]
Declaration Date			Jan. 27, 2016
Date Paid (Payable)			Feb. 26, 2016
Shareholder of Record Date			Feb. 12, 2016
Dividend Amount			$ 0.355