Investments - Equity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Equity investments	$ 729,721	$ 367,125
Publicly traded equity investments in common and preferred stocks
Investment [Line Items]
Equity investments	327,875	138,415
Exchange-traded funds
Investment [Line Items]
Equity investments	133,047	0
Privately held equity investments in common and preferred stocks
Investment [Line Items]
Equity investments	268,799	$ 228,710
AmTrust Financial Services, Inc.
Investment [Line Items]
Indirect investment	$ 240,100