Declared Common Stock Dividends (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014
Dividend Payment 1st
Dividends Payable [Line Items]
Declaration Date	May 11, 2012
Record Date	Jun. 08, 2012
Payable Date	Jun. 22, 2012
Ordinary Income	$ 0.200000	[1]
Total Per Share	$ 0.20
Dividend Payment 2nd
Dividends Payable [Line Items]
Declaration Date	Aug. 20, 2012
Record Date	Sep. 14, 2012
Payable Date	Sep. 28, 2012
Ordinary Income	$ 0.200000	[1]
Total Per Share	$ 0.20
Dividend Payment 3rd
Dividends Payable [Line Items]
Declaration Date	Nov. 06, 2012
Record Date	Nov. 30, 2012
Payable Date	Dec. 14, 2012
Ordinary Income	$ 0.200000	[1]
Total Per Share	$ 0.20
Dividend Payment 4th
Dividends Payable [Line Items]
Declaration Date	Feb. 22, 2013
Record Date	Apr. 03, 2013
Payable Date	Apr. 15, 2013
Ordinary Income	$ 0.346119
Return of Capital	$ 0.183881
Total Per Share	$ 0.53
Dividend Payment 5th
Dividends Payable [Line Items]
Declaration Date	May 16, 2013
Record Date	Jul. 03, 2013
Payable Date	Jul. 15, 2013
Ordinary Income	$ 0.313466
Return of Capital	$ 0.166534
Total Per Share	$ 0.48
Dividend Payment 6th
Dividends Payable [Line Items]
Declaration Date	Aug. 16, 2013
Record Date	Oct. 02, 2013
Payable Date	Oct. 15, 2013
Ordinary Income	$ 0.313466
Return of Capital	$ 0.166534
Total Per Share	$ 0.48
Dividend Payment 7th
Dividends Payable [Line Items]
Declaration Date	Dec. 12, 2013
Record Date	Jan. 02, 2014
Payable Date	Jan. 15, 2014
Ordinary Income	$ 0.480000	[2]
Total Per Share	$ 0.48
Dividend Payment 8th
Dividends Payable [Line Items]
Declaration Date	Feb. 20, 2014
Record Date	Apr. 02, 2014
Payable Date	Apr. 15, 2014
Ordinary Income	$ 0.510000	[3]
Total Per Share	$ 0.51
Dividend Payment 9th
Dividends Payable [Line Items]
Declaration Date	May 15, 2014
Record Date	Jul. 02, 2014
Payable Date	Jul. 15, 2014
Ordinary Income	$ 0.510000	[3]
Total Per Share	$ 0.51
Dividend Payment 10th
Dividends Payable [Line Items]
Declaration Date	Aug. 14, 2014
Record Date	Oct. 02, 2014
Payable Date	Oct. 15, 2014
Ordinary Income	$ 0.510000	[3]
Total Per Share	$ 0.51
Dividend Payment 11th
Dividends Payable [Line Items]
Declaration Date	Dec. 11, 2014
Record Date	Jan. 02, 2015
Payable Date	Jan. 15, 2015
Ordinary Income	$ 0.000000	[4]
Total Per Share	$ 0.51

[1] The amount constitutes a "Qualified Dividend", as defined by the IRS.

[2] $0.072069 of this amount constitutes a "Qualified Dividend", as defined by the IRS.

[3] $0.076573 of this amount constitutes a "Qualified Dividend", as defined by the IRS.

[4] Taxable in 2015.