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	Dec. 31, 2017	Dec. 31, 2016
Assets
Fair value of assets	$ 183,173	$ 335,307
US government debt securities
Assets
Fair value of assets	5,998	180,323
Equity securities
Assets
Fair value of assets	15,100	23,522
Preferred stock
Assets
Fair value of assets	161,225	129,545
Other
Assets
Fair value of assets	850	1,917
Quoted Prices in Active Markets (Level 1)
Assets
Fair value of assets	10,537	15,544
Quoted Prices in Active Markets (Level 1) | US government debt securities
Assets
Fair value of assets	0	0
Quoted Prices in Active Markets (Level 1) | Equity securities
Assets
Fair value of assets	10,537	15,544
Quoted Prices in Active Markets (Level 1) | Preferred stock
Assets
Fair value of assets	0	0
Quoted Prices in Active Markets (Level 1) | Other
Assets
Fair value of assets	0	0
Significant Other Observable Inputs (Level 2)
Assets
Fair value of assets	11,411	190,218
Significant Other Observable Inputs (Level 2) | US government debt securities
Assets
Fair value of assets	5,998	180,323
Significant Other Observable Inputs (Level 2) | Equity securities
Assets
Fair value of assets	4,563	7,978
Significant Other Observable Inputs (Level 2) | Preferred stock
Assets
Fair value of assets	0	0
Significant Other Observable Inputs (Level 2) | Other
Assets
Fair value of assets	850	1,917
Significant Unobservable Inputs (Level 3)
Assets
Fair value of assets	161,225	129,545
Significant Unobservable Inputs (Level 3) | US government debt securities
Assets
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) | Equity securities
Assets
Fair value of assets	0	0
Significant Unobservable Inputs (Level 3) | Preferred stock
Assets
Fair value of assets	161,225	129,545
Significant Unobservable Inputs (Level 3) | Other
Assets
Fair value of assets	$ 0	$ 0