Balance Sheet Components Balance Sheet Components (Cash, cash equivalents, and restricted cash) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Cash and Cash Equivalents [Abstract]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 384.2	$ 369.9	$ 748.8	$ 1,147.0	$ 748.8
Cash and cash equivalents	330.2	292.1	612.8
Restricted cash	$ 54.0	$ 77.8	$ 136.0