Distributions Paid and Payable - Class F Preferred Stock (Details) - Class F Preferred Stock $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Apr. 30, 2017 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) dividend $ / shares	Dec. 31, 2017 $ / shares
Distributions Paid and Payable
Preferred stock redeemed | shares	16,350,000
Number of monthly distributions paid | dividend		3
Dividends paid per preferred share (in dollars per share)	$ 0.101215	$ 0.414063
Preferred stock dividends paid | $	$ 1.7	$ 3.9
Distributions characterized as ordinary income (in dollars per share)			$ 0.5073368
Total capital gain distribution (in dollars per share)			$ 0.0079412