Tax Characterization of Dividends per Share on Common Shares (Parenthetical) (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Dividends Payable [Line Items]
Dividend declared per share	$ 1.72	$ 1.68	$ 2.04
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
Qualified dividend as defined by the IRS
Dividends Payable [Line Items]
Dividend declared per share	0.051840
Qualified dividend as defined by the IRS
Dividends Payable [Line Items]
Dividend declared per share	$ 0.053074