Investments Schedule of Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 630,115	$ 149,097
Gross Unrealized Gains	368	88
Gross Unrealized Losses	1,941	2,213
Fair Value	628,542	146,972
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	524,663	124,861
Gross Unrealized Gains	151	88
Gross Unrealized Losses	0	0
Fair Value	524,814	124,949
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	102,182	24,236
Gross Unrealized Gains	0	0
Gross Unrealized Losses	1,941	2,213
Fair Value	100,241	22,023
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,270
Gross Unrealized Gains	217
Gross Unrealized Losses	0
Fair Value	$ 3,487