Schedule VI - Supplementary Information Concerning Property-Casualty Insurance Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Deferred policy acquisition costs	$ 556,168	$ 517,364	$ 497,629
Reserves for losses and loss expenses	13,784,430	12,583,249	11,966,448
Unearned premiums	4,073,191	3,656,507	3,359,991
Net premiums earned	6,930,843	6,633,288	6,371,505
Net investment income	583,821	645,614	674,235
Losses and loss expenses incurred:
Current year	4,432,937	4,057,989	3,926,489
Prior years	627	34,079	6,831
Loss reserve discount accretion	35,142	39,048	41,382
Amortization of deferred policy acquisition costs		1,001,611	915,246
Paid losses and loss expenses	3,598,649	3,659,402	3,664,885
Net premiums written	$ 7,262,437	$ 6,863,499	$ 6,433,227