Summary of Significant Accounting Policies - Tax Status of Dividends and Distributions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Ordinary dividends (dollars per share)	$ 0.4529	$ 0.5325	$ 0.2693
Qualified dividends (dollars per share)	0	0	0
Capital gain distribution (dollars per share)	0	0	0
Non-dividend distributions (dollars per share)	0.4571	0.3675	0.6307
Total (dollars per share)	$ 0.9100	$ 0.9000	$ 0.9000