Summary of Significant Accounting Policies (Narrative) (Details) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018 segment shares	Dec. 31, 2017 shares	Dec. 31, 2016 shares	Jan. 01, 2019 USD ($)
Property, Plant and Equipment [Line Items]
Potentially dilutive common shares related to the outstanding preferred stock, stock options, restricted stock units and warrants | shares	14.0	13.6	10.7
Number of operating segments (segment) | segment	1
Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of the assets	2 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of the assets	7 years
Subsequent Event | Accounting Standards Update 2016-02
Property, Plant and Equipment [Line Items]
Right-of-use asset				$ 16.8
Operating lease, liability				$ 31.8