Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income taxes
Income tax expense (benefit)	$ 0	$ 0	$ 0
Federal
Income taxes
Net operating loss carryforwards	33.0	14.1
Net operating loss carryforwards excluding deductions related to the exercise of stock options	0.4
State
Income taxes
Net operating loss carryforwards	$ 32.6	$ 13.9