Declared Common Stock Dividends (Detail) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Dividends Payable [Line Items]
Per Share	$ 0.54	$ 0.51	$ 1.62	$ 1.53
Dividend Payment 1st
Dividends Payable [Line Items]
Declaration Date			Feb. 20, 2014
Record Date			Apr. 02, 2014
Payable Date			Apr. 15, 2014
Per Share			$ 0.51
Dividend Payment 2nd
Dividends Payable [Line Items]
Declaration Date			May 15, 2014
Record Date			Jul. 02, 2014
Payable Date			Jul. 15, 2014
Per Share			$ 0.51
Dividend Payment 3rd
Dividends Payable [Line Items]
Declaration Date			Aug. 14, 2014
Record Date			Oct. 02, 2014
Payable Date			Oct. 15, 2014
Per Share			$ 0.51
Dividend Payment 4th
Dividends Payable [Line Items]
Declaration Date			Dec. 11, 2014
Record Date			Jan. 02, 2015
Payable Date			Jan. 15, 2015
Per Share			$ 0.51
Dividend Payment 5th
Dividends Payable [Line Items]
Declaration Date			Feb. 20, 2015
Record Date			Apr. 02, 2015
Payable Date			Apr. 15, 2015
Per Share			$ 0.54
Dividend Payment 6th
Dividends Payable [Line Items]
Declaration Date			May 14, 2015
Record Date			Jul. 02, 2015
Payable Date			Jul. 15, 2015
Per Share			$ 0.54
Dividend Payment 7th
Dividends Payable [Line Items]
Declaration Date			Aug. 13, 2015
Record Date			Oct. 02, 2015
Payable Date			Oct. 15, 2015
Per Share			$ 0.54