Cash and Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Cash and cash equivalents and short-term investments
Cash and cash equivalents, fair value	$ 436,675	$ 501,149
Available-for-sale debt securities:
Total short-term investments	33,222	35,020
Total cash, cash equivalents and short-term investments, fair value	469,897	536,169
Total cash, cash equivalents and short-term investments, gross unrealized gains	12,851	14,648
Total cash, cash equivalents and short-term investments, gross unrealized losses		(1)
Available-for-sale equity securities
Available-for-sale equities securities
Available-for-sale equity securities, fair value	33,205	35,002
Available-for-sale debt securities:
Available-for-sale securities, gross unrealized holding gains	12,851	14,648
Mortgage-CMO debt securities
Available-for-sale debt securities:
Total available-for-sale debt securities, fair value	17	18
Available-for-sale securities, gross unrealized holding losses		(1)
Total Short-term investments
Available-for-sale debt securities:
Total short-term investments	33,222	35,020
Total short-term investments, gross unrealized gains	$ 12,851	14,648
Total short-term investments, gross unrealized holding losses		$ (1)