Tax Characterization of Dividends per Share on Common Shares (Detail) - $ / shares		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Dividends Payable [Line Items]
Total Per Share		$ 1.72	$ 1.68	$ 2.04
Dividend Payment 1st
Dividends Payable [Line Items]
Declaration Date		Feb. 19, 2016
Record Date		Apr. 01, 2016
Payable Date		Apr. 15, 2016
Ordinary Income	[1]	$ 0.487167
Return of Capital		0.052833
Total Per Share		$ 0.54
Dividend Payment 2nd
Dividends Payable [Line Items]
Declaration Date		May 12, 2016
Record Date		Jul. 01, 2016
Payable Date		Jul. 15, 2016
Ordinary Income	[1]	$ 0.487167
Return of Capital		0.052833
Total Per Share		$ 0.54
Dividend Payment 3rd
Dividends Payable [Line Items]
Declaration Date		Aug. 11, 2016
Record Date		Oct. 03, 2016
Payable Date		Oct. 17, 2016
Ordinary Income	[1]	$ 0.487167
Return of Capital		0.052833
Total Per Share		$ 0.54
Dividend Payment 4th
Dividends Payable [Line Items]
Declaration Date		Dec. 08, 2016
Record Date		Jan. 03, 2017
Payable Date		Jan. 13, 2017
Ordinary Income	[2]	$ 0.363660
Return of Capital		0.056340
Total Per Share		$ 0.42
Dividend Payment 5th
Dividends Payable [Line Items]
Declaration Date		Feb. 17, 2017
Record Date		Apr. 03, 2017
Payable Date		Apr. 17, 2017
Ordinary Income	[2]	$ 0.363660
Return of Capital		0.056340
Total Per Share		$ 0.42
Dividend Payment 6th
Dividends Payable [Line Items]
Declaration Date		May 11, 2017
Record Date		Jul. 03, 2017
Payable Date		Jul. 17, 2017
Ordinary Income	[2]	$ 0.363660
Return of Capital		0.056340
Total Per Share		$ 0.42
Dividend Payment 7th
Dividends Payable [Line Items]
Declaration Date		Aug. 10, 2017
Record Date		Oct. 02, 2017
Payable Date		Oct. 16, 2017
Ordinary Income	[2]	$ 0.363660
Return of Capital		0.056340
Total Per Share		$ 0.42
Dividend Payment 8th
Dividends Payable [Line Items]
Declaration Date		Dec. 07, 2017
Record Date		Jan. 02, 2018
Payable Date		Jan. 15, 2018
Ordinary Income	[3]	$ 0.387446
Return of Capital		0.032554
Total Per Share		$ 0.42
Dividend Payment 9th
Dividends Payable [Line Items]
Declaration Date		Feb. 22, 2018
Record Date		Apr. 02, 2018
Payable Date		Apr. 16, 2018
Ordinary Income	[4]	$ 0.396671
Return of Capital		0.033329
Total Per Share		$ 0.43
Dividend Payment 10th
Dividends Payable [Line Items]
Declaration Date		May 11, 2018
Record Date		Jul. 02, 2018
Payable Date		Jul. 16, 2018
Ordinary Income	[4]	$ 0.396671
Return of Capital		0.033329
Total Per Share		$ 0.43
Dividend Payment 11th
Dividends Payable [Line Items]
Declaration Date		Aug. 16, 2018
Record Date		Oct. 01, 2018
Payable Date		Oct. 15, 2018
Ordinary Income	[4]	$ 0.396671
Return of Capital		0.033329
Total Per Share		$ 0.43
Dividend Payment 12th
Dividends Payable [Line Items]
Declaration Date		Dec. 13, 2018
Record Date		Jan. 02, 2019
Payable Date		Jan. 15, 2019
Total Per Share		$ 0.43

[1] $0.030979 of this amount constitutes a "Qualified Dividend", as defined by the IRS.

[2] $0.000000 of this amount constitutes a "Qualified Dividend", as defined by the IRS.

[3] $0.051840 of this amount constitutes a "Qualified Dividend", as defined by the IRS.

[4] $0.053074 of this amount constitutes a "Qualified Dividend", as defined by the IRS.