Marketable Securities The maturities of our marketable securities available for sale (Details)	9 Months Ended
	Sep. 30, 2012
Commercial Paper [Member] | Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturities of marketable securites, by date	Apr. 05, 2013
Commercial Paper [Member] | Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturities of marketable securites, by date	Apr. 05, 2013
Certificates of Deposit [Member] | Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturities of marketable securites, by date	Oct. 14, 2012
Certificates of Deposit [Member] | Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturities of marketable securites, by date	Feb. 18, 2014
Corporate Bonds/Notes [Member] | Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturities of marketable securites, by date	Oct. 01, 2012
Corporate Bonds/Notes [Member] | Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturities of marketable securites, by date	Jan. 15, 2014
Municipal Bonds [Member] | Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturities of marketable securites, by date	Sep. 01, 2013
Agency Bonds/Notes [Member] | Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturities of marketable securites, by date	Jul. 01, 2013
Agency Bonds/Notes [Member] | Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturities of marketable securites, by date	Jul. 01, 2013