Tax Characterization of Dividends per Share on Common Shares (Parenthetical) (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Dividends Payable [Line Items]
Dividend declared per share	$ 1.76	$ 1.72	$ 1.68
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
Qualified dividend as defined by the IRS
Dividends Payable [Line Items]
Dividend declared per share	0.041313
Qualified dividend as defined by the IRS
Dividends Payable [Line Items]
Dividend declared per share	$ 0.042274