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	Mar. 31, 2018	Dec. 31, 2017
Assets
Fair value of financial assets measured at fair value on a recurring basis	$ 180,881	$ 183,173
U.S. government debt securities
Assets
Fair value of financial assets measured at fair value on a recurring basis		5,998
Equity securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	14,043	15,100
Preferred stock
Assets
Fair value of financial assets measured at fair value on a recurring basis	166,069	161,225
Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	769	850
Quoted Prices in Active Markets (Level 1)
Assets
Fair value of financial assets measured at fair value on a recurring basis	9,622	10,537
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
Quoted Prices in Active Markets (Level 1) | Equity securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	9,622	10,537
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	5,190	11,411
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		5,998
Significant Other Observable Inputs (Level 2) | Equity securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	4,421	4,563
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	769	850
Significant Unobservable Inputs (Level 3)
Assets
Fair value of financial assets measured at fair value on a recurring basis	166,069	161,225
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
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	166,069	161,225
Significant Unobservable Inputs (Level 3) | Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	$ 0	$ 0