Cash and Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Cash and cash equivalents and short-term investments
Cash and cash equivalents, fair value	$ 251,366	$ 501,149
Available-for-sale debt securities:
Total short-term investments	25,196	35,020
Total cash, cash equivalents and short-term investments, fair value	276,562	536,169
Available-for-sale equity securities
Available-for-sale equities securities
Available-for-sale equity securities, fair value	25,180	35,002
Available-for-sale debt securities:
Available-for-sale securities, gross unrealized holding gains	6,535	14,648
Available-for-sale securities, gross unrealized holding losses	(1,709)
Mortgage-CMO debt securities
Available-for-sale debt securities:
Total available-for-sale debt securities, fair value	16	18
Available-for-sale securities, gross unrealized holding losses		(1)
Total Short-term investments
Available-for-sale debt securities:
Available-for-sale securities, gross unrealized holding gains	6,535	14,648
Available-for-sale securities, gross unrealized holding losses	(1,709)	(1)
Total short-term investments	$ 25,196	$ 35,020