Fair Value of Financial Instruments -Fair Value Hierarchy (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	$ 95,151	$ 94,843
Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	95,151	94,843
Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
Fair Value | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	95,151	94,843
Commercial Paper
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	55,571	32,376
Commercial Paper | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
Commercial Paper | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	55,571	32,376
Commercial Paper | Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
Commercial Paper | Fair Value | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	55,571	32,376
US Government Agencies
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	8,715	62,467
US Government Agencies | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
US Government Agencies | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	8,715	62,467
US Government Agencies | Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
US Government Agencies | Fair Value | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	8,715	62,467
Municipal bonds
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	1,126
Municipal bonds | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0
Municipal bonds | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	1,126
Municipal bonds | Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0
Municipal bonds | Fair Value | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	1,126
Corporate bonds
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	25,741
Corporate bonds | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0
Corporate bonds | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	25,741
Corporate bonds | Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0
Corporate bonds | Fair Value | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	25,741
Asset backed securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	3,998
Asset backed securities | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0
Asset backed securities | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	3,998
Asset backed securities | Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0
Asset backed securities | Fair Value | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	$ 3,998