Schedule VI - Supplementary Information Concerning Property-Casualty Insurance Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Deferred policy acquisition costs	$ 517,364	$ 497,629	$ 507,549
Reserves for losses and loss expenses	12,583,249	11,966,448	11,670,408
Unearned premiums	3,656,507	3,359,991	3,290,180
Net premiums earned	6,633,288	6,371,505	6,311,419
Net investment income	645,614	674,235	575,788
Losses and loss expenses incurred:
Current year	4,057,989	3,926,489	3,963,543
Prior years	34,079	6,831	(5,165)
Loss reserve discount accretion	39,048	41,382	43,970
Amortization of deferred policy acquisition costs	1,001,611	915,246	1,111,489
Paid losses and loss expenses	3,659,402	3,664,885	3,589,955
Net premiums written	$ 6,863,499	$ 6,433,227	$ 6,260,508