Description of Business and Accounting Policies - Accounting Pronouncements Not Yet Adopted (Details) - USD ($) $ in Billions	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Finance Lease Reclassification		$ 7.5	$ 5.4
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Right-of-Use Asset	$ 21.0
Finance Lease Reclassification	1.2
Operating Lease, Liability	(21.0)
Derecognition of Build-to-suit assets	1.5
Derecognition of Build-to-suit liabilities	$ 1.5