Segment Reporting Disclosures (Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Assets	$ 28,519,411	$ 25,875,299
Non-Insurance Operations [Member]
Segment Reporting Information [Line Items]
Assets	2,106,984	1,448,019
Segment Assets [Member]
Segment Reporting Information [Line Items]
Assets	26,412,427	24,427,280
Segment Assets [Member] | Investing [Member]
Segment Reporting Information [Line Items]
Assets	19,884,334	19,029,584
Segment Assets [Member] | Underwriting [Member]
Segment Reporting Information [Line Items]
Assets	$ 6,528,093	$ 5,397,696