Stockholders' Equity and Non-Controlling Interests (Details) - USD ($) $ / shares in Units, $ in Thousands													1 Months Ended			12 Months Ended				24 Months Ended
	Dec. 09, 2020	Sep. 16, 2020	Jun. 16, 2020	Feb. 25, 2020	Nov. 08, 2019	Aug. 07, 2019	May 08, 2019	Feb. 28, 2019	Nov. 09, 2018	Aug. 08, 2018	May 04, 2018	Feb. 28, 2018	Feb. 29, 2020	Dec. 31, 2019	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Nov. 30, 2019	May 31, 2014
Stockholders' Equity
Dividend declared (in dollars per share)	$ 0.48	$ 0.48	$ 0.48	$ 0.48	$ 0.48	$ 0.48	$ 0.48	$ 0.48	$ 0.48	$ 0.48	$ 0.48	$ 0.48				$ 1.92		$ 1.92	$ 1.92
Shares issued under ATM Agreement																0	0	0
Authorized amount of share repurchases													$ 400,000
Number of shares that may be issued under the DRIP Plan																						11,000,000.0
Value of shares that may be issued under the ATM Agreement																						$ 500,000
Period for repurchase of common stock													1 year
Common stock repurchased (in shares)																2,268,551		573,255
Repurchase of common stock																$ 33,828		$ 12,090
Debt repurchased amount																0		0		$ 0
Remaining capacity to repurchase common stock																366,200
Net income attributable to non-controlling interests																34,392	$ 27,271	25,367
Investment securities														$ 810,238		736,658	810,238
Accrued interest receivable														$ 64,087		95,980	64,087
Amount of non-controlling interest already held by a purchaser of a property																		300
Payment to acquire non-controlling interest																$ 99,512	$ 49,958	$ 256,404
2019 Notes
Stockholders' Equity
Shares issued to settle redemption																	3,600,000	12,400,000
Value of shares issued to settle redemption																	$ 78,000	$ 263,400
Class A Units
Stockholders' Equity
Number of shares of redemption																500,000		0
Number of shares settled in common stock																400,000	1,000,000.0
Number of shares settled in cash																100,000
Cash amount settled																$ 1,300
CMBS JV
Stockholders' Equity
Equity Method Investment, Ownership Percentage														51.00%			51.00%
Net income attributable to non-controlling interests																9,300
Assets sold to joint venture														$ 333,000			$ 333,000
Investment securities														318,300			318,300				$ 24,500
Investments in existing CMBS JV														13,300			13,300
Accrued interest receivable														1,400			1,400
Amount of equity method investment funded														169,800
Non-controlling interest														$ 175,600		$ 126,700	$ 175,600				$ 11,200
CMBS JV | Joint Venture Partner
Stockholders' Equity
Equity Method Investment, Ownership Percentage														49.00%			49.00%
Amount of equity method investment funded														$ 163,200
Woodstar II Portfolio | Class A Units
Stockholders' Equity
Shares issued																100,000	100,000	1,700,000
Right to receive additional shares																		1,900,000
Net income attributable to non-controlling interests																$ 20,400	$ 21,600	$ 17,600
Woodstar II Portfolio | Class A Units | SPT Dolphin
Stockholders' Equity
Shares issued																		7,403,731	10,183,505	10,200,000
Right to receive additional shares																1,910,563		1,411,642	1,910,563	1,900,000
Number of common stock per unit																1			1
Redemption of units																493,318	974,176	0
Woodstar II Portfolio | Class A Units | SPT Dolphin | Non-Controlling Interests
Stockholders' Equity
Redemption of units																226,700,000	235,900,000
REIS Equity Portfolio
Stockholders' Equity
Amount of non-controlling interest already held by a purchaser of a property															$ 300
Payment to acquire non-controlling interest															$ 3,300