Commercial Mortgage Loans - Allocation by Risk Rating (Details) - Commercial Mortgage Receivable, Held-For-Investment $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | loan	118	122
Par Value | $	$ 2,670,133	$ 2,771,299
1
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | loan	0	0
Par Value | $	$ 0	$ 0
2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | loan	90	113
Par Value | $	$ 2,094,654	$ 2,452,330
3
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | loan	27	8
Par Value | $	$ 518,404	$ 298,994
4
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | loan	1	1
Par Value | $	$ 57,075	$ 19,975
5
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | loan	0	0
Par Value | $	$ 0	$ 0