Reinsurance - Narrative (Details) $ in Millions	9 Months Ended
Sep. 30, 2020 USD ($)
Ceded Credit Risk [Line Items]
Insured financial obligations, net par exposures reassumed	$ 336
Increase in unearned premium reserve	5
Commutation gain	38
Puerto Rico
Ceded Credit Risk [Line Items]
Insured financial obligations, net par exposures reassumed	$ 118

Amount of increase (decrease) unearned premiums written. Excludes portion of unearned premiums amortized into income.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

Name:	us-gaap_IncreaseDecreaseInUnearnedPremiums
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration