Related Party Transactions - Summary of Loans (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
HMUS [Member]
Related Party Transaction [Line Items]
Loans and leases receivable from related parties	$ 1,890	$ 6,690
Other short-term affiliate lending [Member]
Related Party Transaction [Line Items]
Loans and leases receivable from related parties	5	60
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Loans and leases receivable from related parties	$ 1,895	$ 6,750

For an unclassified balance sheet, reflects the carrying amount of unpaid loan amounts due from related parties at the balance sheet date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(e))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_LoansAndLeasesReceivableRelatedParties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant