Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2012

Marketable Securities [Abstract]

Available-for-sale Securities [Table Text Block]

At December 31, 2011 and 2012, marketable securities are categorized as follows:

	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Fair Value
December 31, 2011
Municipal notes and bonds	$	70,475	$	60	$	(10	)	$	70,525
Auction rate securities	500		—		(137		)	363
Total marketable securities	$	70,975	$	60	$	(147	)	$	70,888
December 31, 2012
Municipal notes and bonds	$	64,088	$	36	$	(7	)	$	64,117
Corporate Bonds	500		—		—			500
Auction rate securities	475		—		(129		)	346
Total marketable securities	$	65,063	$	36	$	(136	)	$	64,963

Investments Classified by Contractual Maturity Date [Table Text Block]

The amortized cost and estimated fair value of marketable securities classified by the maturity date listed on the security, regardless of the Consolidated Balance Sheet classification, is as follows at December 31, 2011 and 2012:

	December 31, 2011				December 31, 2012
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Due within one year	$	59,469	$	59,509	$	57,367	$	57,390
Due after one through five years	10,587		10,599		7,016		7,022
Due after five through ten years	314		312		205		205
Due after ten years	605		468		475		346
Total marketable securities	$	70,975	$	70,888	$	65,063	$	64,963

Schedule of Unrealized Loss on Investments [Table Text Block]

The following table summarizes the estimated fair value and gross unrealized holding losses of marketable securities, aggregated by investment instrument and period of time in an unrealized loss position at December 31, 2011 and 2012.

	In Unrealized Loss Position For Less Than 12 Months					In Unrealized Loss Position For Greater Than 12 Months
	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
December 31, 2011
Municipal notes and bonds	$	7,291	$	(10	)	$	—	$	—
Tax-free auction rate securities	—		—			363		(137		)
Total marketable securities	$	7,291	$	(10	)	$	363	$	(137	)
December 31, 2012
Municipal notes and bonds	$	9,037	$	(7	)	$	—	$	—
Corporate bonds	—		—			—		—
Tax-free auction rate securities	—		—			346		(129		)
Total marketable securities	$	9,037	$	(7	)	$	346	$	(129	)