Financing Activities (Distributions and Capital Contributions) (Details) (USD $)	1 Months Ended	3 Months Ended		6 Months Ended
	May 31, 2014	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Distributions [Line Items]
Quarterly common stock dividend declaration date	2014-05
Dividend payable date of quarterly common stock dividend	Jul. 01, 2014
Current quarterly common stock dividend (in dollars per share)	$ 0.3725	$ 0.3725	$ 0.3675	$ 0.7450	$ 0.7350
Annualized current quarterly common stock dividend (in dollars per share)	$ 1.49