Stockholders’ Equity - (Narrative) (Details) - USD ($)														1 Months Ended					9 Months Ended	12 Months Ended					23 Months Ended	71 Months Ended
	Oct. 04, 2019	Sep. 09, 2019	Apr. 10, 2019	Mar. 26, 2019	Jan. 09, 2019	Oct. 10, 2018	Jul. 19, 2018	Jul. 03, 2018	Jul. 01, 2018	May 01, 2018	Mar. 27, 2018	Feb. 15, 2018	Jul. 01, 2017	May 31, 2019	Mar. 31, 2019	May 31, 2018	Mar. 27, 2018	Apr. 30, 2013	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2018	Jun. 30, 2018
Class of Stock [Line Items]
Common stock, outstanding (in shares)																				108,475,266	106,230,901					106,230,901	105,058,793
Common stock repurchases (in shares)																					(412,939)	(1,225,365)	(7,854)	(1,769,738)	(303,907)	(3,719,803)
Listing fees																				$ 0	$ 4,988,000	$ 0
Common stock, par value (in dollars per share)								$ 0.01												$ 0.01	$ 0.01					$ 0.01
Common stock, temporary par value, reverse stock split (in dollars per share)								$ 0.02
Common stock issued through distribution reinvestment plan (in shares)																				1,000,000.0
Issuance of new shares																				$ 31,601,000		$ 917,046,000
Preferred stock, authorized (in shares)																				8,796,000	0					0
Preferred stock dividend rate																				7.50%
Preferred stock liquidation preference (in dollars per share)																				$ 25.00
Dividends/Distributions declared on common stock (in dollars per share)									$ 1.10				$ 1.30					$ 1.65		1.10	$ 1.10	$ 1.47
Dividends declared monthly rate (in dollars per share)									$ 0.0916667
Dividends declared on preferred stock (in dollars per share)																				$ 1.875
Reverse stock split ratio								0.5
Initial MacKenzie Offering
Class of Stock [Line Items]
Sale of stock, price per share (in usd per share)										$ 15.35		$ 13.66
Sale of stock, number of shares issued in transaction (in shares)										1,000,000
MacKenzie Offering Counter Offer
Class of Stock [Line Items]
Sale of stock, price per share (in usd per share)										$ 15.45		$ 14.35
Sale of stock, number of shares issued in transaction (in shares)											483,716
MacKenzie Final Offer
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)																207,713
Sale of stock, consideration received on transaction																$ 3,200,000	$ 6,900,000
Maximum
Class of Stock [Line Items]
Preferred stock, authorized (in shares)																				50,000,000
Maximum | Initial MacKenzie Offering
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)												1,000,000
Maximum | MacKenzie Offering Counter Offer
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)										1,000,000		1,000,000
Common Stock
Class of Stock [Line Items]
Common stock, outstanding (in shares)																				108,500,000	106,200,000					106,200,000
Common stock repurchases (in shares)																				(19,870)	(1,142,190)	(1,225,365)
Common stock issued through distribution reinvestment plan (in shares)																					990,393	2,373,256
Issuance of new shares																				$ 22,000		$ 382,000
Class A
Class of Stock [Line Items]
Common stock, outstanding (in shares)																					79,966,215					79,966,215
Authorized repurchase amount								$ 200,000,000.0
Percentage of outstanding stock reclassified								50.00%
Class A | Listed within 90 days
Class of Stock [Line Items]
Percentage of common stock listed							50.00%
Class A | Class A Common Stock ATM Program
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)																				2,229,647
Sale of stock, consideration received on transaction																				$ 31,600,000
Sale Of stock, gross consideration received on transaction																				32,400,000
Commission paid																				$ 800,000
Class A | Maximum | Class A Common Stock ATM Program
Class of Stock [Line Items]
Issuance of new shares														$ 200,000,000.0
Class A | Common Stock
Class of Stock [Line Items]
Common stock, outstanding (in shares)																				80,000,000.0
Class B-1
Class of Stock [Line Items]
Common stock, outstanding (in shares)																				0
Stock dividend rate as a percentage of outstanding shares								50.00%
Class B-1 | Listed initially
Class of Stock [Line Items]
Percentage of common stock listed							25.00%
Class B2
Class of Stock [Line Items]
Common stock, outstanding (in shares)																				0	26,264,686					26,264,686
Stock dividend rate as a percentage of outstanding shares								50.00%
Class B2 | Listed initially
Class of Stock [Line Items]
Percentage of common stock listed							25.00%
Class B2 | Common Stock
Class of Stock [Line Items]
Common stock, outstanding (in shares)																				26,200,000
Series A Cumulative Redeemable Perpetual Preferred Stock
Class of Stock [Line Items]
Preferred stock, authorized (in shares)																				8,796,000
Preferred stock dividend rate															7.50%					7.50%
Redemption period after delisting event																				90 days
Redemption period after change of control																			120 days
Dividends declared on preferred stock (in dollars per share)																				$ 1.875
Series A Cumulative Redeemable Perpetual Preferred Stock | Underwriting Offering, Series A Preferred Stock
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		3,450,000	146,000	1,200,000
Sale of stock, consideration received on transaction		$ 83,500,000		$ 28,600,000
Sale Of stock, gross consideration received on transaction		$ 87,100,000		$ 30,000,000.0
Preferred stock liquidation preference (in dollars per share)		$ 25.25		$ 25.00
Preferred stock redemption price (in dollars per share)																				$ 25.00
Series A Cumulative Redeemable Perpetual Preferred Stock | Series A preferred Stock ATM Program
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)																				2,121,230
Sale of stock, consideration received on transaction																				$ 53,200,000
Sale Of stock, gross consideration received on transaction																				54,000,000.0
Commission paid																				(800,000)
Series A Cumulative Redeemable Perpetual Preferred Stock | Over-Allotment Option
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		450,000
Sale of stock, consideration received on transaction			$ 3,500,000
Sale Of stock, gross consideration received on transaction			$ 3,700,000
Series A Cumulative Redeemable Perpetual Preferred Stock | Maximum | Series A preferred Stock ATM Program
Class of Stock [Line Items]
Issuance of new shares	$ 100,000,000.0													$ 50,000,000.0
Fractional Shares Repurchased As Result Of Automatic Conversion | Class B-1
Class of Stock [Line Items]
Common stock repurchases (in shares)						(19,945)
Fractional Shares Repurchased As Result Of Automatic Conversion | Class B2
Class of Stock [Line Items]
Common stock repurchases (in shares)					(19,870)
Sale of stock, price per share (in usd per share)					$ 13.78
Revolving Credit Facility
Class of Stock [Line Items]
Unused borrowing capacity																				232,800,000
Debt covenant cash and borrowing availability required																				$ 40,000,000.0