Income Tax Expense (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 03, 2016
Operating Loss Carryforwards [Line Items]
U.S. Federal statutory rate to net income (loss) before taxes	34.00%
Deferred tax assets, valuation allowance	$ 118,551	$ 133,784
Change in valuation allowance	(15,200)
Net change in valuation allowance, recorded as tax expense	(24,976)	35,678	$ 10,052
Decrease in net operating loss carryforwards	(101,423)	(124,881)
Decrease in research and development credits	(16,079)	(21,374)
Undistributed earnings of foreign subsidiaries	(100)
Provision for income taxes	$ (459)	$ 498	$ (9,337)
Effective tax rate	0.69%
Provisional tax benefit, Tax reform act	$ 400
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards, domestic	314,000
Internal Revenue Service (IRS) | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforward amount	9,400
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards, state and local	173,000
State and Local Jurisdiction | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforward amount	14,300
Income Tax Expense (Benefit)
Operating Loss Carryforwards [Line Items]
Net change in valuation allowance, recorded as tax expense	(25,000)
TriVascular Technologies, Inc. | Income Tax Expense (Benefit)
Operating Loss Carryforwards [Line Items]
Net change in valuation allowance, recorded as tax expense	$ 9,800
Scenario, Adjustment | TriVascular Technologies, Inc.
Operating Loss Carryforwards [Line Items]
Decrease in research and development credits				$ 3,100
Scenario, Adjustment | TriVascular Technologies, Inc. | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Decrease in net operating loss carryforwards				209,400
Scenario, Adjustment | TriVascular Technologies, Inc. | Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Decrease in net operating loss carryforwards				$ 230,300