Schedule VI. Supplementary Insurance Information Concerning Property-Casualty Insurance Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Line Items]
Current year	$ 1,390,767	$ 1,902,424	$ 694,957
Prior years	(270,749)	(40,996)	(164,126)
Consolidated Subsidiaries
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs	476,661	426,551	335,325
Reserves for Unpaid Claims and Claim Adjustment Expenses	6,076,271	5,080,408	2,848,294
Discount, if any, Deducted	0	0	0
Unearned Premiums	1,716,021	1,477,609	1,231,573
Earned Premiums	1,976,129	1,717,575	1,403,430
Net Investment Income	261,866	222,209	181,726
Current year	1,390,767	1,902,424	694,957
Prior years	(270,749)	(40,996)	(164,126)
Amortization of Deferred Policy Acquisition Costs	432,989	346,892	289,323
Paid Claims and Claim Adjustment Expenses	894,769	974,825	589,294
Net Premiums Written	$ 2,131,902	$ 1,871,325	$ 1,535,312