Fair Value Measurements - Summary of Placement in the Fair Value Hierarchy of Financial Assets that are Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets
Fair value of financial assets measured at fair value on a recurring basis	$ 303,898	$ 280,497
U.S. government debt securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	226,858	115,335
Equity Securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	76,623	164,889
Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	417	273
Quoted Prices in Active Markets (Level 1)
Assets
Fair value of financial assets measured at fair value on a recurring basis	61,747	143,927
Quoted Prices in Active Markets (Level 1) | U.S. government debt securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets (Level 1) | Equity Securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	61,747	143,927
Quoted Prices in Active Markets (Level 1) | Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	0	0
Significant Other Observable Inputs (Level 2)
Assets
Fair value of financial assets measured at fair value on a recurring basis	242,151	136,570
Significant Other Observable Inputs (Level 2) | U.S. government debt securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	226,858	115,335
Significant Other Observable Inputs (Level 2) | Equity Securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	14,876	20,962
Significant Other Observable Inputs (Level 2) | Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	417	273
Significant Unobservable Inputs (Level 3)
Assets
Fair value of financial assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | U.S. government debt securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Equity Securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	$ 0	$ 0