Schedule II - Condensed Financial Information of Registrant (Statements of Cash Flows (Parent Company)) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Cash flows from operating activities:
Net income	$ 394,803	$ 449,287	$ 309,057
Adjustments to reconcile net income to net cash from operating activities:
Depreciation and amortization	88,012	82,867	78,875
Stock incentive plans	27,175	27,407	24,465
Change in:
Other liabilities	10,269	19,514	123,687
Net cash from operating activities	670,279	451,316	316,054
Cash from (used in) investing activities:
Proceeds from sales of fixed maturity securities	1,293,876	1,554,906	2,436,258
Proceeds from maturities and prepayments of fixed maturity securities	1,697,144	1,374,378	1,214,157
Proceeds from sales of equity securities	159,827	137,990	188,646
Cost of purchases of fixed maturity securities	(2,815,340)	(2,755,449)	(4,869,368)
Investment in funds	(113,913)	(53,979)	(87,011)
Change in balance due to security broker	(16,194)	(297)	144,023
Net additions to property, furniture & equipment	(45,320)	(49,605)	(30,455)
Net cash from (used in) investing activities	(204,274)	37,347	(1,116,234)
Cash from (used in) financing activities:
Net proceeds from issuance of senior notes	0	309,030	333,589
Net proceeds from stock options exercised	21,963	17,730	5,426
Purchase of common treasury shares	(187,163)	(471,007)	(147,144)
Cash dividends to common stockholders	(43,253)	(49,348)	(28,843)
Other, net	14,550	(2,795)	21,522
Net cash from (used in) financing activities	(195,213)	(359,075)	173,385
Net increase (decrease) in cash and cash equivalents	268,790	127,522	(619,405)
Cash and cash equivalents at beginning of year	642,952	515,430	1,134,835
Cash and cash equivalents at end of year	911,742	642,952	515,430
Parent Company [Member]
Cash flows from operating activities:
Net income	394,803	449,287	309,057
Adjustments to reconcile net income to net cash from operating activities:
Net investment (gains) losses	(45,962)	1,891	(20,961)
Depreciation and amortization	4,905	3,963	2,279
Equity in undistributed (earnings) losses of subsidiaries	(74,738)	(372,577)	(245,370)
Tax payments received from subsidiaries	139,011	106,284	103,356
Federal income taxes provided by subsidiaries on a separate return basis	(79,200)	(28,377)	(117,133)
Stock incentive plans	27,176	26,318	24,078
Change in:
Federal income taxes	56,542	29,332	62,753
Other assets	(542)	16,430	(381)
Other liabilities	(6,747)	11,467	11,661
Accrued investment income	1,559	(2,776)	(137)
Other, net	277	0	(603)
Net cash from operating activities	417,084	241,242	128,599
Cash from (used in) investing activities:
Proceeds from sales of fixed maturity securities	70,665	164,920	29,355
Proceeds from maturities and prepayments of fixed maturity securities	165,158	85,695	47,133
Proceeds from sales of equity securities	47,735	3	17,897
Cost of purchases of fixed maturity securities	(240,536)	(195,646)	(353,944)
Investment in funds	0	0	5,204
Investments in and advances to subsidiaries, net	(3,867)	(18,685)	(29,179)
Change in balance due to security broker	(5,983)	(8,500)	14,483
Net additions to property, furniture & equipment	(643)	(1,212)	(224)
Other, net	0	0	1
Net cash from (used in) investing activities	32,529	26,575	(269,274)
Cash from (used in) financing activities:
Net proceeds from issuance of senior notes	0	296,636	297,461
Net proceeds from stock options exercised	21,966	17,730	5,426
Repayments of senior notes	0	(150,000)	0
Purchase of common treasury shares	(187,163)	(471,007)	(147,144)
Cash dividends to common stockholders	(43,255)	(49,348)	(28,843)
Other, net	0	0	0
Net cash from (used in) financing activities	(208,452)	(355,989)	126,900
Net increase (decrease) in cash and cash equivalents	241,161	(88,172)	(13,775)
Cash and cash equivalents at beginning of year	29,476	117,648	131,423
Cash and cash equivalents at end of year	$ 270,637	$ 29,476	$ 117,648