Mortgage-Backed Securities and other securities - Unconsolidated CMBS VIEs (Details) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan
Variable Interest Entity [Line Items]
Number of commercial loans trusts	3	3
Variable interest entity, not primary beneficiary, aggregated disclosure
Variable Interest Entity [Line Items]
Number of commercial loans trusts	8	10
Variable interest entity, reporting entity involvement, maximum loss exposure, amount \| $	$ 74.6	$ 117.7

The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5747-111685

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5728-111685

Name:	us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant