Equity (Details) $ / shares in Units, $ in Thousands					1 Months Ended	3 Months Ended		12 Months Ended
	May 02, 2018 $ / shares	Feb. 21, 2018 $ / shares	Feb. 02, 2018 $ / shares	Jul. 14, 2016 USD ($) shares	Mar. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2017 $ / shares	Dec. 31, 2017 $ / shares shares	Dec. 31, 2016 shares	Dec. 31, 2015
Common stock
Proceeds from issuance of shares under public offering						$ 3,014
Noncontrolling Interest
Preferred stock, par value (in dollars per share) | $ / shares					$ 0.01	$ 0.01		$ 0.01
Distributions
Cash dividend declared (in dollars per share) | $ / shares						$ 0.21	$ 0.17
Common Stock
Common stock
Number of shares available under an "At-The-Market" equity offering with JMP Securities LLC | shares					7,140,000	7,140,000
Chief executive officer | Restricted common stock
Deferred Compensation
Number of fully vested shares issued | shares						63,584
Total grant date fair value						$ 600
Vesting percentage						25.00%
Chief executive officer | Restricted common stock | First Anniversaries
Deferred Compensation
Vesting percentage						25.00%
Chief executive officer | Restricted common stock | Second Anniversaries
Deferred Compensation
Vesting percentage						25.00%
Chief executive officer | Restricted common stock | Third Anniversaries
Deferred Compensation
Vesting percentage						25.00%
Chief executive officer | Performance
Deferred Compensation
Granted of restricted stock units that vest at the end of four-year performance period (in shares) | shares						381,503
Vesting period (in years)						4 years		4 years	4 years	4 years
Total grant date fair value						$ 800
Chief executive officer | Employee compensation and benefits | Restricted common stock
Deferred Compensation
Share-based compensation expense						100
Chief executive officer | Employee compensation and benefits | Performance | Maximum
Deferred Compensation
Share-based compensation expense						$ 100
Employees | Restricted common stock
Deferred Compensation
Number of fully vested shares issued | shares					265,444
Total grant date fair value					$ 2,300
Vesting percentage					33.00%
Employees | Restricted common stock | March, 2019
Deferred Compensation
Vesting percentage					33.00%
Employees | Restricted common stock | March 2020
Deferred Compensation
Vesting percentage					33.00%
Employees | Employee compensation and benefits | Restricted common stock
Deferred Compensation
Share-based compensation expense					$ 800
8.25% Series A preferred stock
Preferred Stock
Return on the preferred shares issued to third parties by its subsidiary REIT (as a percent)						8.25%	8.25%	8.25%
7.75% Series B preferred stock
Preferred Stock
Return on the preferred shares issued to third parties by its subsidiary REIT (as a percent)						7.75%	7.75%	7.75%
8.50% Series C preferred stock
Preferred Stock
Return on the preferred shares issued to third parties by its subsidiary REIT (as a percent)						8.50%	8.50%	8.50%
Common Stock
Common stock
Number of shares to be issued under the equity offering sales agreement | shares					7,500,000	7,500,000
Number of common stock sold under public offering (in shares) | shares						360,000
Proceeds from issuance of shares under public offering						$ 3,000
Value of common stock available under shelf registration statement					$ 176,600	176,600
Aggregate amount of securities filed under shelf registration statement					$ 500,000	$ 500,000
Distributions
Cash dividend declared (in dollars per share) | $ / shares	$ 0.25	$ 0.21
Common Stock | Non-management members of the Board of Directors
Deferred Compensation
Number of fully vested shares issued | shares					58,620
Common Stock | Non-management members of the Board of Directors | Selling and administrative expense
Deferred Compensation
Share-based compensation expense					$ 500
Preferred Stock | 8.25% Series A preferred stock
Distributions
Cash dividend declared on redeemable preferred stock (in dollars per share) | $ / shares	$ 0.515625		$ 0.515625
Preferred Stock
Return on the preferred shares issued to third parties by its subsidiary REIT (as a percent)	8.25%
Preferred Stock | 7.75% Series B preferred stock
Distributions
Cash dividend declared on redeemable preferred stock (in dollars per share) | $ / shares	$ 0.484375		0.484375
Preferred Stock
Return on the preferred shares issued to third parties by its subsidiary REIT (as a percent)	7.75%
Preferred Stock | 8.50% Series C preferred stock
Distributions
Cash dividend declared on redeemable preferred stock (in dollars per share) | $ / shares	$ 0.53125		$ 0.53125
Preferred Stock
Return on the preferred shares issued to third parties by its subsidiary REIT (as a percent)	8.50%
ACM Acquisition | ACM / Our "Former Manager"
Noncontrolling Interest
Consideration in stock to be paid with operating partnership units (in shares) | shares						15,450,421		5,780,348	21,230,769
Number of preferred stock shares paired with each OP units | shares									1
Operating Partnership Units
Noncontrolling Interest
Consideration in stock to be paid with operating partnership units (in shares) | shares				21,230,769
Value of operating partnership units issues as consideration				$ 154,800
Conversion ratio for operating partnership units to common stock shares						1
Operating Partnership Units | Special voting preferred shares
Noncontrolling Interest
Number of preferred stock shares paired with each OP units | shares						1
Preferred stock, par value (in dollars per share) | $ / shares					$ 0.01	$ 0.01
Voting power of outstanding stock (as a percent)						25.40%