Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Narrative) (Details) - Accounting Standards Update 2016-09 [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Summary of Significant Accounting Policies [Line Items]
Excess tax benefits recognized	$ 20.0
Excess Tax Benefits [Member]
Summary of Significant Accounting Policies [Line Items]
Decrease in net cash used for operations and net cash provided by financing activities	20.0	$ 15.0
Employee Taxes Paid [Member]
Summary of Significant Accounting Policies [Line Items]
Decrease in net cash used for operations and net cash provided by financing activities	$ 26.0	$ 23.0