Schedule II. Condensed Financial Information of Registrant (Condensed Balance Sheets) (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets:
Short-term investments	$ 38,285	$ 63,288
Total investments	19,847,271	17,118,544
Cash	75,933	121,026
Other assets	583,933	549,899
Total assets	25,977,460	23,095,722
Liabilities:
Short-term debt	298,738	307,848
Long-term debt	1,348,988	1,357,185
Other liabilities	525,068	453,270
Total liabilities	18,683,153	17,680,545
Shareholders' equity:
Preferred stock	0	0
Common stock	117,218	121,218
Additional paid-in-capital	531,554	524,414
Accumulated other comprehensive income	1,844,830	319,475
Retained earnings	5,551,329	5,213,468
Treasury stock	(750,624)	(763,398)
Total shareholders' equity	7,294,307	5,415,177	$ 6,231,421	$ 4,566,861
Total liabilities and shareholders' equity	25,977,460	23,095,722
Parent Company
Assets:
Long-term investments	28,728	29,603
Short-term investments	3,401	21
Total investments	32,129	29,624
Cash	873	760
Investment in affiliates	9,020,073	7,128,588
Due from affiliates	96,129	96,110
Taxes receivable from affiliates	50,302	50,656
Other assets	160,723	152,103
Total assets	9,360,229	7,457,841
Liabilities:
Short-term debt	298,738	307,848
Long-term debt	1,498,851	1,507,000
Due to affiliates	3,532	3,002
Other liabilities	264,801	224,814
Total liabilities	2,065,922	2,042,664
Shareholders' equity:
Preferred stock	351	351
Common stock	117,218	121,218
Additional paid-in-capital	882,065	874,925
Accumulated other comprehensive income	1,844,830	319,475
Retained earnings	5,551,329	5,213,468
Treasury stock	(1,101,486)	(1,114,260)
Total shareholders' equity	7,294,307	5,415,177
Total liabilities and shareholders' equity	$ 9,360,229	$ 7,457,841