Financing Activities (Distributions and Capital Contributions) (Details) (USD $)	1 Months Ended	3 Months Ended
	Feb. 28, 2015	Mar. 31, 2015	Mar. 31, 2014
Distributions [Line Items]
Quarterly common stock dividend declaration date	2015-02
Dividend payable date of quarterly common stock dividend	Apr. 01, 2015
Current quarterly common stock dividend (in dollars per share)	$ 0.3725	$ 0.3725	$ 0.3725
Annualized current quarterly common stock dividend (in dollars per share)	$ 1.49