Schedule VI. Supplementary Insurance Information Concerning Property-Casualty Insurance Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Current year	$ 1,902,424	$ 694,957	$ 610,685
Prior years	(40,996)	(164,126)	(162,447)
Consolidated Subsidiaries
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs	426,551	335,325	199,380
Reserves for Unpaid Claims and Claim Adjustment Expenses	5,080,408	2,848,294	2,767,045
Discount, if any, Deducted	0	0	0
Unearned Premiums	1,477,609	1,231,573	889,102
Earned Premiums	1,717,575	1,403,430	1,400,551
Net Investment Income	222,209	181,726	152,567
Current year	1,902,424	694,957	610,685
Prior years	(40,996)	(164,126)	(162,447)
Amortization of Deferred Policy Acquisition Costs	346,892	289,323	238,592
Paid Claims and Claim Adjustment Expenses	974,825	589,294	521,312
Net Premiums Written	$ 1,871,325	$ 1,535,312	$ 1,416,183