Commercial Loans - Commercial loans held (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		$ 2,676,231
Fair Value		2,691,532	$ 2,493,238	$ 368,972	$ 216,361
Securitized Commercial Loans | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		279,425
Fair Value		279,425
Securitized Commercial Loans | RSBC Trust
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		90,788
Fair Value		90,788
CRE One | Interest-Only Mezzanine loan | Securitized Commercial Loans | Hotel | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		20,000
Fair Value		$ 20,000
Coupon Rate		71.00%
CRE Two | Interest-Only First Mortgage | Securitized Commercial Loans | Hotel | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		$ 30,000
Fair Value		$ 30,000
Coupon Rate		65.00%
CRE Four | Principal & Interest First Mortgage | Securitized Commercial Loans | Nursing Facilities | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		$ 50,000
Fair Value		$ 50,000
Coupon Rate		75.00%
CRE Five | Interest-Only Mezzanine loan | Securitized Commercial Loans | Entertainment and Retail | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		$ 90,000
Fair Value		$ 90,000
Coupon Rate		58.00%
CRE Six | Interest-Only First Mortgage | Securitized Commercial Loans | Retail | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		$ 40,000
Fair Value		$ 40,000
Coupon Rate		63.00%
CRE Seven | Interest-Only First Mortgage | Securitized Commercial Loans | Hotel | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		$ 24,535
Fair Value		$ 24,535
Coupon Rate		62.00%
CRE Eight | Interest-Only First Mortgage | Securitized Commercial Loans | Hotel | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		$ 13,206
Fair Value		$ 13,206
Coupon Rate		62.00%
CRE Nine | Interest-Only First Mortgage | Securitized Commercial Loans | Hotel | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		$ 7,259
Fair Value		$ 7,259
Coupon Rate		62.00%
CRE Ten | Interest-Only First Mortgage | Securitized Commercial Loans | Assisted Living | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		$ 4,425
Fair Value		$ 4,425
Coupon Rate		79.00%
SBC 1 | Interest-Only First Mortgage | Securitized Commercial Loans | Nursing Facilities | RSBC Trust
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		$ 45,188
Fair Value		$ 45,188
Coupon Rate		74.00%
SBC 4 | Interest-Only First Mortgage | Securitized Commercial Loans | Apartment Complex | RSBC Trust
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		$ 13,600
Fair Value		$ 13,600
Coupon Rate		84.00%
SBC 5 | Interest-Only First Mortgage | Securitized Commercial Loans | Nursing Facilities | RSBC Trust
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Principal balance		$ 32,000
Fair Value		$ 32,000
Coupon Rate		49.00%
London Interbank Offered Rate (LIBOR) | CRE One | Interest-Only Mezzanine loan | Securitized Commercial Loans | Hotel | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate		6.50%
London Interbank Offered Rate (LIBOR) | CRE Two | Interest-Only First Mortgage | Securitized Commercial Loans | Hotel | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate		4.50%
London Interbank Offered Rate (LIBOR) | CRE Four | Principal & Interest First Mortgage | Securitized Commercial Loans | Nursing Facilities | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate		4.75%
London Interbank Offered Rate (LIBOR) | CRE Five | Interest-Only Mezzanine loan | Securitized Commercial Loans | Entertainment and Retail | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate		9.25%
London Interbank Offered Rate (LIBOR) | CRE Six | Interest-Only First Mortgage | Securitized Commercial Loans | Retail | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate		3.02%
London Interbank Offered Rate (LIBOR) | CRE Seven | Interest-Only First Mortgage | Securitized Commercial Loans | Hotel | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate		3.75%
London Interbank Offered Rate (LIBOR) | CRE Eight | Interest-Only First Mortgage | Securitized Commercial Loans | Hotel | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate		3.75%
London Interbank Offered Rate (LIBOR) | CRE Nine | Interest-Only First Mortgage | Securitized Commercial Loans | Hotel | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate		3.75%
London Interbank Offered Rate (LIBOR) | CRE Ten | Interest-Only First Mortgage | Securitized Commercial Loans | Assisted Living | CRE LLC And CRE Mezz
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate		4.85%
London Interbank Offered Rate (LIBOR) | SBC 1 | Interest-Only First Mortgage | Securitized Commercial Loans | Nursing Facilities | RSBC Trust
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate		4.25%
London Interbank Offered Rate (LIBOR) | SBC 4 | Interest-Only First Mortgage | Securitized Commercial Loans | Apartment Complex | RSBC Trust
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate		4.00%
London Interbank Offered Rate (LIBOR) | SBC 5 | Interest-Only First Mortgage | Securitized Commercial Loans | Nursing Facilities | RSBC Trust
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate		4.10%
London Interbank Offered Rate (LIBOR) Floor | SBC 1 | Interest-Only First Mortgage | Securitized Commercial Loans | Nursing Facilities | RSBC Trust
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate	1.25%
London Interbank Offered Rate (LIBOR) Floor | SBC 4 | Interest-Only First Mortgage | Securitized Commercial Loans | Apartment Complex | RSBC Trust
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Coupon Rate	2.00%