Investments Investments (Components Of Restricted Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Components Of Restricted Assets [Line Items]
Restricted assets	$ 4,561,710	$ 5,163,830
Assets held in trust or on deposit to support underwriting activities [Member]
Components Of Restricted Assets [Line Items]
Restricted assets	4,155,621	4,780,613
Assets pledged as security for letters of credit pledged as collateral [Member]
Components Of Restricted Assets [Line Items]
Restricted assets	$ 406,089	$ 383,217