Tax Treatment of Distributions - Dividends (Details) - $ / shares			3 Months Ended		12 Months Ended
	Jan. 13, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Dividends declared per common share (in dollars per share)			$ 0.375	$ 0.375	$ 1.91000	$ 1.79	$ 1.65
Dividends paid per common share (in dollars per share)	$ 0.313
Common Stock [Member]
Class of Stock [Line Items]
Dividends declared per common share (in dollars per share)				2.275	1.910	1.790	1.650
Less: Dividends declared in the current year and paid in the following year (in dollars per share)					(0.485)	(0.455)	(0.425)
Add: Dividends declared in the prior year and paid in the current year (in dollars per share)					0.455	0.425	2.275
Dividends paid per common share (in dollars per share)					$ 1.880	$ 1.760	$ 3.500
Special Dividend [Member]
Class of Stock [Line Items]
Dividends declared per common share (in dollars per share)				$ 1.90
Dividends paid per common share (in dollars per share)		$ 1.587