Financing Activities - Distributions (Details) - $ / shares	1 Months Ended	3 Months Ended		6 Months Ended
	May 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Distributions (Numeric) [Abstract]
Dividend payable date of quarterly common stock dividend	Jul. 02, 2018
Current quarterly common stock dividend (in dollars per share)	$ 0.410	$ 0.41	$ 0.395	$ 0.82	$ 0.79
Annualized current quarterly common stock dividend (in dollars per share)	$ 1.64