Equity - Tax Treatment of Dividends Paid (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends Payable [Line Items]
Tax treatment of capital gain dividends, percent	100.00%	100.00%	100.00%
Capital gains
Dividends Payable [Line Items]
Tax treatment of capital gain dividends, percent	9.06%	0.00%	0.00%
Ordinary income
Dividends Payable [Line Items]
Tax treatment of capital gain dividends, percent	71.59%	78.00%	64.20%
Return of capital
Dividends Payable [Line Items]
Tax treatment of capital gain dividends, percent	19.35%	22.00%	35.80%