Reinsurance Reinsurance (Effect of Reinsurance and Retrocessional Reinsurance on Incurred Losses and Loss Adjustment Expenses) (Details) - Underwriting Operations [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effects of Reinsurance [Line Items]
Gross	$ 3,447,186	$ 3,530,790	$ 3,722,604
Ceded losses and loss adjustment expenses	(556,618)	(710,568)	(856,843)
Net losses and loss adjustment expenses	$ 2,890,568	$ 2,820,222	$ 2,865,761