Schedule II. Condensed Financial Information of Registrant (Condensed Balance Sheets) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets:
Short-term investments	$ 127,071	$ 72,040
Total investments	17,734,484	15,879,728
Cash	118,563	76,163	$ 61,383	$ 66,019
Other assets	528,536	520,313
Total assets	23,474,985	21,436,087
Liabilities:
Short-term debt	328,067	264,475
Long-term debt	1,132,201	1,133,165
Other liabilities	489,609	413,760
Total liabilities	17,243,564	16,869,226
Shareholders' equity:
Preferred stock	0	0
Common stock	124,218	127,218
Additional paid-in capital	508,476	490,421
Accumulated other comprehensive income	1,424,274	577,574
Retained earnings	4,806,208	3,890,798
Treasury stock	(631,755)	(519,150)
Total shareholders’ equity	6,231,421	4,566,861	4,055,552	4,697,466
Total liabilities and shareholders’ equity	23,474,985	21,436,087
Parent Company
Assets:
Long-term investments	35,562	33,586
Short-term investments	5,624	0
Total investments	41,186	33,586
Cash	1,008	0	$ 0	$ 0
Investment in affiliates	7,763,704	6,004,429
Due from affiliates	95,920	96,005
Taxes receivable from affiliates	63,099	88,406
Other assets	135,616	119,801
Total assets	8,100,533	6,342,227
Liabilities:
Short-term debt	328,067	264,475
Long-term debt	1,281,971	1,282,891
Due to affiliates	8,002	0
Other liabilities	251,072	228,000
Total liabilities	1,869,112	1,775,366
Shareholders' equity:
Preferred stock	351	351
Common stock	124,218	127,218
Additional paid-in capital	858,987	840,932
Accumulated other comprehensive income	1,424,274	577,574
Retained earnings	4,806,208	3,890,798
Treasury stock	(982,617)	(870,012)
Total shareholders’ equity	6,231,421	4,566,861
Total liabilities and shareholders’ equity	$ 8,100,533	$ 6,342,227