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, 2016	Dec. 31, 2015
Assets
Fair value of financial assets measured at fair value on a recurring basis	$ 376,093	$ 291,762
U.S. government debt securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	210,689	207,704
Equity securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	39,432	83,653
Preferred stock
Assets
Fair value of financial assets measured at fair value on a recurring basis	123,676
Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	2,296	405
Quoted Prices in Active Markets (Level 1)
Assets
Fair value of financial assets measured at fair value on a recurring basis	31,245	65,850
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	31,245	65,850
Quoted Prices in Active Markets (Level 1) | Preferred stock
Assets
Fair value of financial assets measured at fair value on a recurring basis	0
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	221,172	225,912
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	210,689	207,704
Significant Other Observable Inputs (Level 2) | Equity securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	8,187	17,803
Significant Other Observable Inputs (Level 2) | Preferred stock
Assets
Fair value of financial assets measured at fair value on a recurring basis	0
Significant Other Observable Inputs (Level 2) | Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	2,296	405
Significant Unobservable Inputs (Level 3)
Assets
Fair value of financial assets measured at fair value on a recurring basis	123,676	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) | Preferred stock
Assets
Fair value of financial assets measured at fair value on a recurring basis	123,676
Significant Unobservable Inputs (Level 3) | Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	$ 0	$ 0