Declared Common Stock Dividends (Detail) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Dividends Payable [Line Items]
Per Share	$ 0.42	$ 0.54	$ 1.26	$ 1.62
Dividend Payment 1st
Dividends Payable [Line Items]
Declaration Date			Feb. 19, 2016
Record Date			Apr. 01, 2016
Payable Date			Apr. 15, 2016
Per Share			$ 0.54
Dividend Payment 2nd
Dividends Payable [Line Items]
Declaration Date			May 12, 2016
Record Date			Jul. 01, 2016
Payable Date			Jul. 15, 2016
Per Share			$ 0.54
Dividend Payment 3rd
Dividends Payable [Line Items]
Declaration Date			Aug. 11, 2016
Record Date			Oct. 03, 2016
Payable Date			Oct. 17, 2016
Per Share			$ 0.54
Dividend Payment 4th
Dividends Payable [Line Items]
Declaration Date			Dec. 08, 2016
Record Date			Jan. 03, 2017
Payable Date			Jan. 13, 2017
Per Share			$ 0.42
Dividend Payment 5th
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 6th
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 7th
Dividends Payable [Line Items]
Declaration Date			Aug. 10, 2017
Record Date			Oct. 02, 2017
Payable Date			Oct. 16, 2017
Per Share			$ 0.42