Concentration of Risks and Uncertainties (Details) (USD $)	Mar. 31, 2014	Mar. 31, 2014 Single Corporate Issuer	Apr. 30, 2014 Single Corporate Issuer Subsequent Event
Concentration Risk [Line Items]
U.S. treasury securities	$ 434,800,000
Corporate debt securities	$ 278,417,000	$ 48,000,000	$ 81,900,000