Investments - Amortized Costs and Estimated Fair Values (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Schedule of Investments [Line Items]
Total investments, amortized cost	$ 156,092		$ 109,313
Total investments, gross unrealized gains	24		9
Total investments, gross unrealized losses	(39)		(3)
Total investments, fair value	156,077		109,319
Realized investment gains (losses)	0		0
Total available for sale marketable securities [Member]
Schedule of Investments [Line Items]
Marketable Securities	150,182		105,665
Available-for-sale securities, amortized cost	150,197		105,659
Available-for-sale securities, gross unrealized gains	24		9
Available-for-sale securities, gross unrealized losses	(39)		(3)
Available-for-sale securities, fair value	150,182		105,665
U.S. treasury notes [Member]
Schedule of Investments [Line Items]
Marketable Securities	10,128		60,786
Available-for-sale securities, amortized cost	10,130		60,785
Available-for-sale securities, gross unrealized gains	0		4
Available-for-sale securities, gross unrealized losses	(2)		(3)
Available-for-sale securities, fair value	10,128		60,786
Agency bonds [Member]
Schedule of Investments [Line Items]
Marketable Securities	37,961		31,157
Available-for-sale securities, amortized cost	37,996	[1]	31,153	[1]
Available-for-sale securities, gross unrealized gains	0	[1]	4	[1]
Available-for-sale securities, gross unrealized losses	(35)	[1]	0	[1]
Available-for-sale securities, fair value	37,961	[1]	31,157	[1]
Commercial paper [Member]
Schedule of Investments [Line Items]
Marketable Securities	39,987		5,000
Available-for-sale securities, amortized cost	39,987		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	39,987		5,000
Certificates of deposit [Member]
Schedule of Investments [Line Items]
Marketable Securities	7,786		5,494
Available-for-sale securities, amortized cost	7,786		5,494
Available-for-sale securities, gross unrealized gains	0		0
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, fair value	7,786		5,494
Municipal bonds [Member]
Schedule of Investments [Line Items]
Marketable Securities	49,319		3,228
Available-for-sale securities, amortized cost	49,296		3,227
Available-for-sale securities, gross unrealized gains	24		1
Available-for-sale securities, gross unrealized losses	(1)		0
Available-for-sale securities, fair value	49,319		3,228
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Marketable Securities	5,001		0
Available-for-sale securities, amortized cost	5,002
Available-for-sale securities, gross unrealized gains	0
Available-for-sale securities, gross unrealized losses	(1)
Available-for-sale securities, fair value	5,001
Equity securities - mutual funds [Member]
Schedule of Investments [Line Items]
Marketable Securities	5,287		3,046
Trading securities, amortized cost	5,287		3,046
Trading securities, gross unrealized gains	0		0
Trading securities, gross unrealized losses	0		0
Trading securities, fair value	5,287		3,046
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.