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, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Cash Flow Statements, Captions [Line Items]
Fixed maturity securities available for sale at fair value (cost $272,283 and $61,084 at December 31, 2014 and 2013, respectively)	$ 12,705,160	$ 11,616,844
Cash flows from operating activities:
Net income	648,884	499,925	510,592
Adjustments to reconcile net income to net cash from operating activities:
Net investment gains	(254,852)	(121,544)	(210,465)
Depreciation and amortization	88,836	103,090	103,419
Stock incentive plans	28,068	23,784	26,763
Change in:
Other liabilities	(58,254)	42,787	102,014
Net cash from operating activities	734,847	819,798	675,458
Cash from (used in) investing activities:
Proceeds from sale of fixed maturity securities	633,459	1,344,707	838,576
Proceeds from maturities and prepayments of fixed maturity securities	2,605,839	2,718,156	2,291,105
Proceeds from sales of equity securities	113,251	267,554	409,610
Cost of purchases of fixed maturity securities	(4,292,165)	(4,198,135)	(3,617,002)
Payments to Acquire Available-for-sale Securities, Equity	(31,207)	(156,557)	(284,991)
Payments to Acquire Businesses, Net of Cash Acquired	(65,421)	(56,878)	(42,779)
Change in balance due to security broker	32,617	(26,155)	(25,799)
Net additions to real estate, furniture & equipment	(41,958)	(63,150)	(40,556)
Net cash used in investing activities	(808,311)	(545,364)	(775,068)
Cash from (used in) financing activities:
Net proceeds from stock options exercised	0	53	7,660
Purchase of common treasury shares	(238,933)	(166,473)	(127,663)
Cash dividends to common stockholders	(181,489)	(52,717)	(183,947)
Net cash from (used in) financing activities	(69,773)	(330,262)	97,192
Net decrease in cash and cash equivalents	(165,297)	(65,932)	(6,072)
Cash and cash equivalents at beginning of year	839,738	905,670	911,742
Cash and cash equivalents at end of year	674,441	839,738	905,670
Parent Company
Condensed Cash Flow Statements, Captions [Line Items]
Fixed maturity securities available for sale at fair value (cost $272,283 and $61,084 at December 31, 2014 and 2013, respectively)	273,773	63,115
Cash flows from operating activities:
Net income	648,884	499,925	510,592
Adjustments to reconcile net income to net cash from operating activities:
Net investment gains	(5,487)	(24,550)	(71,130)
Depreciation and amortization	2,916	10,397	5,624
Equity in undistributed earnings of subsidiaries	(307,401)	(369,640)	(694,701)
Tax payments received from subsidiaries	462,809	77,305	125,046
Federal income taxes provided by subsidiaries on a separate return basis	(366,721)	(225,845)	(132,379)
Stock incentive plans	28,068	23,784	26,763
Change in:
Federal income taxes	(15,239)	21,866	44,423
Other assets	(364)	(821)	911
Other liabilities	(39,780)	(15,470)	28,017
Accrued investment income	(820)	967	97
Other, net	0	0	(13)
Net cash from operating activities	406,865	(2,082)	(156,750)
Cash from (used in) investing activities:
Proceeds from sale of fixed maturity securities	289,683	230,854	93,354
Proceeds from maturities and prepayments of fixed maturity securities	103,646	68,918	121,191
Proceeds from sales of equity securities	7,356	23,395	72,565
Cost of purchases of fixed maturity securities	(605,768)	(79,132)	(255,885)
Payments to Acquire Available-for-sale Securities, Equity	0	(4,668)	0
Payments to Acquire Businesses, Net of Cash Acquired	(82,879)	0	0
Investments in and advances to subsidiaries, net	34,191	(58,454)	(16,480)
Change in balance due to security broker	(2,151)	6,918	(4,767)
Net additions to real estate, furniture & equipment	(1,615)	(1,896)	(1,857)
Net cash used in investing activities	(257,537)	185,935	8,121
Cash from (used in) financing activities:
Net proceeds from issuance of senior notes	344,472	339,627	345,823
Net proceeds from stock options exercised	0	53	13,544
Repayment of senior notes	0	(450,000)	0
Purchase of common treasury shares	(238,933)	(166,473)	(134,456)
Cash dividends to common stockholders	(181,489)	(52,717)	(183,947)
Net cash from (used in) financing activities	(75,950)	(329,510)	40,964
Net decrease in cash and cash equivalents	73,378	(145,657)	(107,665)
Cash and cash equivalents at beginning of year	17,315	162,972
Cash and cash equivalents at end of year	$ 90,693	$ 17,315	$ 162,972