Income Taxes - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Nov. 03, 2014
Income Tax Disclosure [Abstract]
Federal statutory tax rate	34.00%	34.00%	34.00%
Operating Loss Carryforwards [Line Items]
Net increase to valuation allowance primarily due to additional net operating losses	$ 15.5	$ 22.4	$ 25.3
Share-based compensation tax benefit not recognized in deferred tax assets, net of allowance	39.4
Share-based compensation tax benefit not recognized in deferred tax assets	107.6
Undistributed foreign earnings	4.0	2.2
Potential tax impact if undistributed foreign earnings were distributed	0.8	$ 0.3
Reduction in tax expense if unrecognized tax benefits are recognized	0.4
Federal Tax Authority
Operating Loss Carryforwards [Line Items]
Federal, state and foreign net operating losses	254.7
Federal Tax Authority | Evolv Inc.
Operating Loss Carryforwards [Line Items]
Deferred tax assets, tax credit carryforwards, research				$ 0.4
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Federal, state and foreign net operating losses	255.6
State and Local Jurisdiction | Evolv Inc.
Operating Loss Carryforwards [Line Items]
Deferred tax assets, tax credit carryforwards, research				$ 0.5
Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Federal, state and foreign net operating losses	$ 59.6