Reserve for Known and Incurred but Not Reported Claims (Activity in Reserve for Known and Incurred but Not Reported Claims) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance at beginning of period	$ 1,178,004	$ 1,063,044
Provision related to current year	134,820	103,213
Provision related to prior years	5,627	14,264
Total Provision	140,447	117,477
Payments, net of recoveries, related to: Current year	38,384	32,685
Payments, net of recoveries, related to: Prior years	79,824	75,853
Total Payments, net of recoveries	118,208	108,538
Other	2,926	(14,445)
Balance at end of period	$ 1,203,169	$ 1,057,538