Subsequent Events (Narrative) (Details) - $ / shares			3 Months Ended						9 Months Ended
	Oct. 06, 2015	Oct. 06, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Sep. 30, 2015
Subsequent Event [Line Items]
Dividend paid to common stockholders			$ 0.32	$ 0.32	$ 0.29	$ 0.29	$ 0.29	$ 0.24
Dividends payable, date of record			Jun. 19, 2015				Jun. 20, 2014
Dividends payable, date payable			Jul. 08, 2015	Apr. 07, 2015	Jan. 07, 2015	Jul. 08, 2014	Apr. 08, 2014	Jan. 07, 2014
Qualitytech, LP [Member]
Subsequent Event [Line Items]
Dividends payable, date of record									Sep. 18, 2015
Subsequent Event [Member] | Qualitytech, LP [Member]
Subsequent Event [Line Items]
Dividend paid to common stockholders		$ 0.32
Dividends payable, date payable	Oct. 06, 2015