Mortgage-Backed Securities and other securities - Unconsolidated CMBS VIEs (Details) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2019 loan	Dec. 31, 2019 USD ($) entity	Dec. 31, 2018 USD ($) loan
Schedule of Investments [Line Items]
VIE, consolidated, number of commercial loan trusts \| entity		4
Variable interest entity, reporting entity involvement, maximum loss exposure, amount \| $		$ 117.7	$ 118.4
Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure
Schedule of Investments [Line Items]
VIE, consolidated, number of commercial loan trusts \| loan	10		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=108788376&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=108788376&loc=d3e5728-111685

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