Investments - Schedule of Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 284,340	$ 149,097
Gross Unrealized Gains	119	88
Gross Unrealized Losses	2,669	2,213
Fair Value	281,790	146,972
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Cost	224,826	124,861
Gross Unrealized Gains	16	88
Gross Unrealized Losses	0	0
Fair Value	224,842	124,949
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost	56,244	24,236
Gross Unrealized Gains	0	0
Gross Unrealized Losses	2,669	2,213
Fair Value	53,575	22,023
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Cost	3,270
Gross Unrealized Gains	103
Gross Unrealized Losses	0
Fair Value	$ 3,373