Leases (Narrative) (Details) $ in Billions	Sep. 30, 2018 USD ($)
Property Subject to or Available for Operating Lease [Line Items]
Minimum sublease rents	$ 1.5
Lease term (in years)	20 years
Minimum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease renewal option as lessee, minimum (in years)	1 year
Lease renewal option as lessor, minimum (in years)	5 years
Maximum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease renewal option as lessee, minimum (in years)	20 years
Lease renewal option as lessor, minimum (in years)	20 years