Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income taxes
Income tax expense (benefit)	$ 0	$ 0	$ 0
Federal
Income taxes
Net operating loss carryforwards	72.1	33.0	14.1
Net operating loss carryforwards excluding deductions related to the exercise of stock options	5.5
State
Income taxes
Net operating loss carryforwards	71.5	32.6	13.9
Stock options | State
Income taxes
Net operating loss carryforwards	$ 5.5