Tax Characterization of Dividends per Share on Common Shares (Detail) - $ / shares		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Dividends Payable [Line Items]
Total Per Share		$ 0.44	$ 1.76	$ 1.72
Dividend Payment 1st
Dividends Payable [Line Items]
Declaration Date		Feb. 22, 2018
Record Date		Apr. 02, 2018
Payable Date		Apr. 16, 2018
Ordinary Income	[1]	$ 0.396671
Return of Capital		0.033329
Total Per Share		$ 0.43
Dividend Payment 2nd
Dividends Payable [Line Items]
Declaration Date		May 11, 2018
Record Date		Jul. 02, 2018
Payable Date		Jul. 16, 2018
Ordinary Income	[1]	$ 0.396671
Return of Capital		0.033329
Total Per Share		$ 0.43
Dividend Payment 3rd
Dividends Payable [Line Items]
Declaration Date		Aug. 16, 2018
Record Date		Oct. 01, 2018
Payable Date		Oct. 15, 2018
Ordinary Income	[1]	$ 0.396671
Return of Capital		0.033329
Total Per Share		$ 0.43
Dividend Payment 4th
Dividends Payable [Line Items]
Declaration Date		Dec. 13, 2018
Record Date		Jan. 02, 2019
Payable Date		Jan. 15, 2019
Ordinary Income	[2]	$ 0.374927
Return of Capital		0.055073
Total Per Share		$ 0.43
Dividend Payment 5th
Dividends Payable [Line Items]
Declaration Date		Feb. 21, 2019
Record Date		Apr. 01, 2019
Payable Date		Apr. 15, 2019
Ordinary Income	[3]	$ 0.383646
Return of Capital		0.056354
Total Per Share		$ 0.44
Dividend Payment 6th
Dividends Payable [Line Items]
Declaration Date		May 16, 2019
Record Date		Jul. 01, 2019
Payable Date		Jul. 16, 2019
Ordinary Income	[3]	$ 0.383646
Return of Capital		0.056354
Total Per Share		$ 0.44
Dividend Payment 7th
Dividends Payable [Line Items]
Declaration Date		Aug. 15, 2019
Record Date		Oct. 01, 2019
Payable Date		Oct. 15, 2019
Ordinary Income	[3]	$ 0.383646
Return of Capital		0.056354
Total Per Share		$ 0.44
Dividend Payment 8th
Dividends Payable [Line Items]
Declaration Date		Dec. 12, 2019
Record Date		Jan. 06, 2020
Payable Date		Jan. 15, 2020
Ordinary Income	[4]	$ 0.440000
Total Per Share		$ 0.44
Dividend Payment 9th
Dividends Payable [Line Items]
Declaration Date		Feb. 20, 2020
Record Date		Apr. 01, 2020
Payable Date		Apr. 15, 2020
Ordinary Income	[4]	$ 0.440000
Total Per Share		$ 0.44

[1] $0.053074 of this amount constitutes a "Qualified Dividend", as defined by the IRS.

[2] $0.041413 of this amount constitutes a "Qualified Dividend", as defined by the IRS.

[3] $0.042774 of this amount constitutes a "Qualified Dividend", as defined by the IRS.

[4] $0.040745 of this amount constitutes a "Qualified Dividend", as defined by the IRS.