Note 5 - Investment Securities - Available For Sale (Detail) - Short-term Investment by Bond Maturities (USD $)	Mar. 31, 2013	Dec. 31, 2012
Within 3 Months [Member]
Corporate bond maturities
Cost	$ 3,108,805	$ 3,053,740
Fair value	3,003,280	3,002,350
Between 3-12 Months [Member]
Corporate bond maturities
Cost	22,252,626	5,110,487
Fair value	22,199,595	5,035,270
Bond maturities [Member]
Corporate bond maturities
Cost	25,361,431	8,164,227
Fair value	$ 25,202,875	$ 8,037,620

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Debt Securities which consist of all investments in certain debt securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt securities which are categorized as Available-for-sale.

No definition available.

Name:	clsn_AvailableForSaleDebtSecuritiesFairValueDisclosure
Namespace Prefix:	clsn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant