Summary of Significant Accounting Policies Tax Characteristics of Dividends (Details) - Common Stock [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Tax Characterization of Dividends Paid [Line Items]
Percent of Total Dividends Paid	100.00%	100.00%	100.00%
Ordinary Income [Member]
Tax Characterization of Dividends Paid [Line Items]
Percent of Total Dividends Paid	23.70%	72.60%	4.20%
Return of Capital [Member]
Tax Characterization of Dividends Paid [Line Items]
Percent of Total Dividends Paid	0.00%	2.60%	0.00%
Capital Gains [Member]
Tax Characterization of Dividends Paid [Line Items]
Percent of Total Dividends Paid	76.30%	24.80%	95.80%