Financing Activities (Distributions and Capital Contributions) (Details) - $ / shares	1 Months Ended	3 Months Ended		9 Months Ended
	Aug. 31, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Distributions [Line Items]
Quarterly common stock dividend declaration date	2015-08
Dividend payable date of quarterly common stock dividend	Oct. 01, 2015
Current quarterly common stock dividend (in dollars per share)	$ 0.3775	$ 0.3775	$ 0.3725	$ 1.1225	$ 1.1175
Annualized current quarterly common stock dividend (in dollars per share)	$ 1.51