Fair Value of Financial Instruments - Additional Information (Detail)	3 Months Ended
Mar. 31, 2014
Accounting Policies [Abstract]
Maturity period of highly liquid investments	90 days

Cash and cash equivalents maximum maturity period.

No definition available.

Name:	geva_CashAndCashEquivalentsMaximumMaturityPeriod
Namespace Prefix:	geva_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration