Declared Common Stock Dividends (Detail) - $ / shares	3 Months Ended						9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018
Dividends Payable [Line Items]
Per Share	$ 0.43	$ 0.43	$ 0.43	$ 0.42	$ 0.42	$ 0.42
Dividend Payment 1st
Dividends Payable [Line Items]
Declaration Date							Feb. 17, 2017
Record Date							Apr. 03, 2017
Payable Date							Apr. 17, 2017
Per Share							$ 0.42
Dividend Payment 2nd
Dividends Payable [Line Items]
Declaration Date							May 11, 2017
Record Date							Jul. 03, 2017
Payable Date							Jul. 17, 2017
Per Share							$ 0.42
Dividend Payment 3rd
Dividends Payable [Line Items]
Declaration Date							Aug. 10, 2017
Record Date							Oct. 02, 2017
Payable Date							Oct. 16, 2017
Per Share							$ 0.42
Dividend Payment 4th
Dividends Payable [Line Items]
Declaration Date							Dec. 07, 2017
Record Date							Jan. 02, 2018
Payable Date							Jan. 15, 2018
Per Share							$ 0.42
Dividend Payment 5th
Dividends Payable [Line Items]
Declaration Date							Feb. 22, 2018
Record Date							Apr. 02, 2018
Payable Date							Apr. 16, 2018
Per Share							$ 0.43
Dividend Payment 6th
Dividends Payable [Line Items]
Declaration Date							May 11, 2018
Record Date							Jul. 02, 2018
Payable Date							Jul. 16, 2018
Per Share							$ 0.43
Dividend Payment 7th
Dividends Payable [Line Items]
Declaration Date							Aug. 16, 2018
Record Date							Oct. 01, 2018
Payable Date							Oct. 15, 2018
Per Share							$ 0.43