Investments - Amortized Costs and Estimated Fair Values (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Schedule of Investments [Line Items]
Total investments, amortized cost	$ 109,313		$ 70,133
Total investments, gross unrealized gains	9		31
Total investments, gross unrealized losses	(3)		(2)
Total investments, fair value	109,319		70,162
Realized investment gains (losses)	0		0		0
Equity securities - mutual funds [Member]
Schedule of Investments [Line Items]
Trading securities, amortized cost	3,046		2,810
Trading securities, gross unrealized gains	0		0
Trading securities, gross unrealized losses	0		0
Trading securities, fair value	3,046		2,810
Total available for sale marketable securities [Member]
Schedule of Investments [Line Items]
Available-for-sale securities, amortized cost	105,659		66,715
Available-for-sale securities, gross unrealized gains	9		31
Available-for-sale securities, gross unrealized losses	(3)		(2)
Available-for-sale securities, fair value	105,665		66,744
U.S. treasury notes [Member]
Schedule of Investments [Line Items]
Available-for-sale securities, amortized cost	60,785
Available-for-sale securities, gross unrealized gains	4
Available-for-sale securities, gross unrealized losses	(3)
Available-for-sale securities, fair value	60,786
Agency bonds [Member]
Schedule of Investments [Line Items]
Available-for-sale securities, amortized cost	31,153	[1]	58,508	[1]
Available-for-sale securities, gross unrealized gains	4	[1]	27	[1]
Available-for-sale securities, gross unrealized losses	0	[1]	(2)	[1]
Available-for-sale securities, fair value	31,157	[1]	58,533	[1]
Commercial paper [Member]
Schedule of Investments [Line Items]
Available-for-sale securities, amortized cost	5,000		5,000
Available-for-sale securities, gross unrealized gains	0		0
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, fair value	5,000		5,000
Certificates of deposit [Member]
Schedule of Investments [Line Items]
Available-for-sale securities, amortized cost	5,494		3,207
Available-for-sale securities, gross unrealized gains	0		4
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, fair value	5,494		3,211
Municipal bonds [Member]
Schedule of Investments [Line Items]
Available-for-sale securities, amortized cost	3,227
Available-for-sale securities, gross unrealized gains	1
Available-for-sale securities, gross unrealized losses	0
Available-for-sale securities, fair value	3,228
Cost-method investments [Member]
Schedule of Investments [Line Items]
Cost method investments	608	[2]	608	[2]
Cost method investments, gross unrealized gains	0	[2]	0	[2]
Cost method investments, gross unrealized losses	0	[2]	0	[2]
Cost method investments, fair value	$ 608	[2]	$ 608	[2]

[1] Agency bonds are securities backed by U.S. government-sponsored entities.

[2] Investments for which it is not practical to estimate fair value are included at cost and primarily consist of investments in privately-held companies.