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Stockholders' Equity - Additional Information (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	May 31, 2017 USD ($) $ / shares shares	Sep. 30, 2018 $ / shares shares	Sep. 30, 2018 $ / shares shares	Dec. 31, 2017 $ / shares shares	Aug. 10, 2018 shares	May 16, 2017 shares	Feb. 22, 2017 shares	May 24, 2013 shares
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Preferred stock, shares authorized (in shares)		25,000,000	25,000,000	25,000,000
Preferred stock, par value (in dollars per share) \| $ / shares		$ 0.01	$ 0.01	$ 0.01
Prior Equity Distribution Agreements
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Agreement termination date			Feb. 23, 2017
Common Stock
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Repurchase of Class A common stock (in shares)				0
Common Class A
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Common stock, shares authorized (in shares)		450,000,000	450,000,000	450,000,000
Common stock, par value (in dollars per share) \| $ / shares		$ 0.01	$ 0.01	$ 0.01
Common Stock Voting Rights Per Share Owned		1	1
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased		1,951,305	1,951,305
Common Class A \| Maximum
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased		2,000,000	2,000,000
Common Class A \| Common Stock
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Conversion of Class B common stock to Class A common stock (in shares)				20,256
Repurchase of Class A common stock (in shares)		0	0
Issuance of stock (in shares)			2,074,205	4,472,083
Common Class A \| Common Stock \| Amended New Equity Distribution Agreements
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Number of Shares Offer and Sell		11,837,443	11,837,443
Common Class A \| Common Stock \| Maximum
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Number of Shares Offer and Sell								1,750,000
Common Class A \| Common Stock \| Maximum \| New Equity Distribution Agreements
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Number of Shares Offer and Sell							6,000,000
Common Class A \| Common Stock \| Maximum \| Amended New Equity Distribution Agreements
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Number of Shares Offer and Sell					12,597,423
Common Class B
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Common stock, shares authorized (in shares)		100,000,000	100,000,000
Common stock, par value (in dollars per share) \| $ / shares		$ 0.01	$ 0.01
Common Stock Voting Rights Per Share Owned		3	3
Common Class B \| Common Stock
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Conversion of Class B common stock to Class A common stock (in shares)				(20,256)
7.00 % Series B Cumulative Perpetual Redeemable Preferred Stock
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Preferred stock, shares authorized (in shares)		2,000,000	2,000,000
Preferred stock, par value (in dollars per share) \| $ / shares		$ 0.01	$ 0.01
Series A Preferred Stock
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Preferred stock, shares authorized (in shares)		100,000	100,000
Preferred stock shares unissued		100,000	100,000
Undesignated Preferred Stock
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Preferred stock, shares authorized (in shares)		22,900,000	22,900,000
Series B Preferred Stock
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Public Offering Price Per Share \| $ / shares	$ 24.00
Net proceeds underwriting discounts and commissions and expenses \| $	$ 3,018
Preferred stock, dividend rate percentage			7.00%
Preferred stock voting rights per share owned		0	0
Preferred stock, liquidation preference per share \| $ / shares		$ 25.00	$ 25.00
Preferred stock, redeemable price per share \| $ / shares		$ 25.00	$ 25.00
Preferred stock, redemption date			May 12, 2022
Preferred stock, dividend payment terms			Dividends are payable quarterly in arrears on the 30th day of each December, March, June and September.
Series B Preferred Stock \| Initial Public Offering
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Issuance of stock (in shares)	135,000
Series B Preferred Stock \| Preferred Stock
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Issuance of stock (in shares)			42,865	303,291
Series B Preferred Stock \| Preferred Stock \| Series B Preferred Equity Distribution Agreement
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Issuance of stock (in shares)			42,865	168,291
Number of Shares Offer and Sell		1,653,844	1,653,844
Series B Preferred Stock \| Preferred Stock \| Maximum
Share based Compensation Arrangement by Share based Payment Award [Line Items]
Number of Shares Offer and Sell						1,865,000