Summary of Significant Accounting Policies - Leveraged Leases (Narrative) (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)
Leveraged Lease [Line Items]
Reduction in net income after tax based on lease receivable balance	$ 86
Maximum
Leveraged Lease [Line Items]
Original term (up to)	45 years

The net investment in arrangements meeting the criteria to be classified as leveraged excluding deferred taxes.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 50
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82918944&loc=SL77920254-209983

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84165051&loc=d3e45424-112738

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77907235&loc=d3e45152-112736

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