Marketable Securities - Available-for-sale Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Due within one year, Amortized Cost	$ 4,724
Due within one year, Estimated fair value	4,734
Due after one year through five years, Amortized Cost	19,927
Due after one year through five years, Estimated fair value	20,036
Due after five years through ten years, Amortized Cost	1,574
Due after five years through ten years, Estimated fair value	1,609
Total available-for-sale securities, Amortized Cost	26,225	25,490
Total available-for-sale securities, Estimated fair value	$ 26,379	$ 25,526