Tax Treatment of Distributions - Dividends (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Dividends declared per common share (in dollars per share)	$ 1.97	$ 1.91	$ 1.79
Common Stock [Member]
Class of Stock [Line Items]
Dividends declared per common share (in dollars per share)	1.970	1.910	1.790
Less: Dividends declared in the current year and paid in the following year (in dollars per share)	(0.500)	(0.485)	(0.455)
Add: Dividends declared in the prior year and paid in the current year (in dollars per share)	0.485	0.455	0.425
Dividends paid per common share (in dollars per share)	$ 1.955	$ 1.880	$ 1.760