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, 2017	Dec. 31, 2016
Assets
Fair value of financial assets measured at fair value on a recurring basis	$ 226,215	$ 335,307
U.S. government debt securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	43,985	180,323
Equity securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	26,642	23,522
Preferred stock
Assets
Fair value of financial assets measured at fair value on a recurring basis	148,499	129,545
Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	7,089	1,917
Quoted Prices in Active Markets (Level 1)
Assets
Fair value of financial assets measured at fair value on a recurring basis	19,830	15,544
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	19,830	15,544
Quoted Prices in Active Markets (Level 1) | Preferred stock
Assets
Fair value of financial assets measured at fair value on a recurring basis	0	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	57,886	190,218
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	43,985	180,323
Significant Other Observable Inputs (Level 2) | Equity securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	6,812	7,978
Significant Other Observable Inputs (Level 2) | Preferred stock
Assets
Fair value of financial assets measured at fair value on a recurring basis	0	0
Significant Other Observable Inputs (Level 2) | Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	7,089	1,917
Significant Unobservable Inputs (Level 3)
Assets
Fair value of financial assets measured at fair value on a recurring basis	148,499	129,545
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	148,499	129,545
Significant Unobservable Inputs (Level 3) | Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	$ 0	$ 0