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, 2016	Dec. 31, 2015
Assets
Fair value of assets	$ 335,307	$ 291,762
US government debt securities
Assets
Fair value of assets	180,323	207,704
Equity securities
Assets
Fair value of assets	23,522	83,653
Preferred stock
Assets
Fair value of assets	129,545
Other
Assets
Fair value of assets	1,917	405
Quoted Prices in Active Markets (Level 1)
Assets
Fair value of assets	15,544	65,850
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	15,544	65,850
Quoted Prices in Active Markets (Level 1) | Preferred stock
Assets
Fair value of assets	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	190,218	225,912
Significant Other Observable Inputs (Level 2) | US government debt securities
Assets
Fair value of assets	180,323	207,704
Significant Other Observable Inputs (Level 2) | Equity securities
Assets
Fair value of assets	7,978	17,803
Significant Other Observable Inputs (Level 2) | Preferred stock
Assets
Fair value of assets	0
Significant Other Observable Inputs (Level 2) | Other
Assets
Fair value of assets	1,917	405
Significant Unobservable Inputs (Level 3)
Assets
Fair value of assets	129,545	0
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	129,545
Significant Unobservable Inputs (Level 3) | Other
Assets
Fair value of assets	$ 0	$ 0