Fair Value Measurements - Debt Financing (Details) - USD ($) $ in Billions	Jun. 30, 2019	Dec. 31, 2018
Book Value
Financial Instruments Not Measured at Fair Value
Debt financing	$ 13.0	$ 11.7
Level 2
Financial Instruments Not Measured at Fair Value
Debt financing	$ 13.3	$ 11.4

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant