Tax Characterization of Dividends per Share on Common Shares (Parenthetical) (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Dividends Payable [Line Items]
Dividend declared per share	$ 0.44	$ 1.76	$ 1.72
Qualified dividend as defined by the IRS
Dividends Payable [Line Items]
Dividend declared per share	0.053074
Qualified dividend as defined by the IRS
Dividends Payable [Line Items]
Dividend declared per share	0.041413
Qualified dividend as defined by the IRS
Dividends Payable [Line Items]
Dividend declared per share	0.042774
Qualified dividend as defined by the IRS
Dividends Payable [Line Items]
Dividend declared per share	$ 0.040745