Significant Accounting Policies Significant Accounting Policies Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Jan. 01, 2019
Significant Accounting Policies [Line Items]
Operating and Finance Lease Liabilities	$ 1,254
Operating and Finance Lease, Right-of-Use Asset	1,178
Interest Received from Sales-Type and Direct Financing Leases	$ 34
Adoption of ASU 2016-02 [Member]
Significant Accounting Policies [Line Items]
Sale Leaseback Transaction, Deferred Gain, Gross		$ 31
Operating and Finance Lease Liabilities		1,300
Operating and Finance Lease, Right-of-Use Asset		$ 1,200