Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value Measurements
Amount of transfers of financial assets between Level 1 and Level 2 measures	$ 0
Assets:
Short term investments:	96,015	200,577
Available-for-sale equity securities
Assets:
Short term investments:	76,469	174,610
Recurring
Assets:
Total short-term investments	96,015
Recurring | Available-for-sale equity securities
Assets:
Short term investments:	76,469
Recurring | Corporate debt securities
Assets:
Short term investments:	18,448
Recurring | Mortgage-backed debt securities
Assets:
Short term investments:	213
Recurring | Mortgage-CMO debt securities
Assets:
Short term investments:	60
Recurring | Asset-backed debt securities
Assets:
Short term investments:	825
Recurring | Level 1
Assets:
Total short-term investments	76,432
Recurring | Level 1 | Available-for-sale equity securities
Assets:
Short term investments:	75,607
Recurring | Level 1 | Asset-backed debt securities
Assets:
Short term investments:	825
Recurring | Level 2
Assets:
Total short-term investments	19,583
Recurring | Level 2 | Available-for-sale equity securities
Assets:
Short term investments:	862
Recurring | Level 2 | Corporate debt securities
Assets:
Short term investments:	18,448
Recurring | Level 2 | Mortgage-backed debt securities
Assets:
Short term investments:	213
Recurring | Level 2 | Mortgage-CMO debt securities
Assets:
Short term investments:	$ 60