Financing Activities (Distributions and Capital Contributions) (Details) - $ / shares	1 Months Ended		3 Months Ended		6 Months Ended
	Aug. 31, 2015	May. 31, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Distributions [Line Items]
Quarterly common stock dividend declaration date		2015-05
Dividend payable date of quarterly common stock dividend		Jul. 01, 2015
Current quarterly common stock dividend (in dollars per share)		$ 0.3725	$ 0.3725	$ 0.3725	$ 0.745	$ 0.745
Annualized current quarterly common stock dividend (in dollars per share)		$ 1.49
Subsequent Event [Member]
Distributions [Line Items]
Quarterly common stock dividend declaration date	2015-08
Dividend payable date of quarterly common stock dividend	Oct. 01, 2015
Quarterly common stock dividend (in dollars per share)	$ 0.3775
Annualized current quarterly common stock dividend (in dollars per share)	$ 1.51
Date of record	Sep. 10, 2015