Schedule II - Condensed Financial Information Of Registrant (Condensed Balance Sheets) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Assets:
Total investments	$ 14,094,209	$ 12,353,586
Cash	61,710	84,113
Other assets	401,763	391,076
Total assets	18,776,910	16,588,272
Liabilities:
Short-term debt	319,043	224,842
Long-term debt	989,686	790,571
Due to affiliates	0	124,421
Other liabilities	392,502	312,417
Total liabilities	14,415,124	12,728,641
Shareholders' equity:
Preferred stock	0	0
Common stock	105,812	112,312
Additional paid-in capital	439,782	425,331
Accumulated other comprehensive income	925,275	549,916
Retained earnings	3,403,338	3,264,711
Treasury stock	(512,421)	(492,639)
Total shareholders' equity	4,361,786	3,859,631	3,667,329	3,068,043
Total liabilities and shareholders' equity	18,776,910	16,588,272
Parent Company
Assets:
Long-term investments	31,060	36,458
Short-term investments	1,610	58
Total investments	32,670	36,516
Cash	0	27,099
Investment in affiliates	5,780,762	4,960,492
Due from affiliates	156,995	50,977
Taxes receivable	86,391	61,616
Other assets	27,635	7,581
Total assets	6,084,453	5,144,281
Liabilities:
Short-term debt	319,043	224,842
Long-term debt	1,139,253	790,571
Due to affiliates	59,358	145,556
Other liabilities	205,013	123,681
Total liabilities	1,722,667	1,284,650
Shareholders' equity:
Preferred stock	351	351
Common stock	105,812	112,312
Additional paid-in capital	790,293	775,842
Accumulated other comprehensive income	925,275	549,916
Retained earnings	3,403,338	3,264,711
Treasury stock	(863,283)	(843,501)
Total shareholders' equity	4,361,786	3,859,631
Total liabilities and shareholders' equity	$ 6,084,453	$ 5,144,281