Declared Common Stock Dividends (Parenthetical) (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends Payable [Line Items]
Dividend declared per share	$ 1.68	$ 2.04	$ 2.16
Qualified dividend as defined by the IRS
Dividends Payable [Line Items]
Dividend declared per share	0.051202
Qualified dividend as defined by the IRS
Dividends Payable [Line Items]
Dividend declared per share	0.030979
Qualified dividend as defined by the IRS
Dividends Payable [Line Items]
Dividend declared per share	$ 0.000000