Schedule II. Condensed Financial Information of Registrant (Condensed Balance Sheets) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets:
Short-term investments	$ 107,782	$ 38,285
Total investments	22,452,651	19,847,271
Cash	94,847	75,933
Other assets	739,027	583,933
Total assets	29,046,731	25,977,460
Liabilities:
Short-term debt	254,918	298,738
Long-term debt	1,667,886	1,348,988
Other liabilities	716,373	525,068
Total liabilities	20,275,639	18,683,153
Shareholders' equity:
Preferred stock	0	0
Common stock	113,218	117,218
Additional paid-in-capital	527,435	531,554
Accumulated other comprehensive income	3,029,244	1,844,830
Retained earnings	5,874,109	5,551,329
Treasury stock	(772,914)	(750,624)
Total shareholders' equity	8,771,092	7,294,307	$ 5,415,177	$ 6,231,421
Total liabilities and shareholders' equity	29,046,731	25,977,460
Parent Company
Assets:
Long-term investments	32,861	28,728
Short-term investments	19,300	3,401
Total investments	52,161	32,129
Cash	1,644	873
Investment in affiliates	10,526,982	9,020,073
Due from affiliates	322,278	96,129
Taxes receivable from affiliates	51,041	50,302
Other assets	184,588	160,723
Total assets	11,138,694	9,360,229
Liabilities:
Short-term debt	254,918	298,738
Long-term debt	1,817,798	1,498,851
Due to affiliates	0	3,532
Other liabilities	294,886	264,801
Total liabilities	2,367,602	2,065,922
Shareholders' equity:
Preferred stock	351	351
Common stock	113,218	117,218
Additional paid-in-capital	877,946	882,065
Accumulated other comprehensive income	3,029,244	1,844,830
Retained earnings	5,874,109	5,551,329
Treasury stock	(1,123,776)	(1,101,486)
Total shareholders' equity	8,771,092	7,294,307
Total liabilities and shareholders' equity	$ 11,138,694	$ 9,360,229