Schedule II - Condensed Financial Information of Registrant (Statements of Cash Flows (Parent Company)) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Cash Flow Statements, Captions [Line Items]
Fixed maturity securities available for sale at fair value (cost $201,256 and $272,283 at December 31, 2015 and 2014, respectively)	$ 12,444,394				$ 12,705,160				$ 12,444,394	$ 12,705,160
Cash flows from operating activities:
Net income	109,745	$ 152,607	$ 123,035	$ 118,307	110,711	$ 188,539	$ 179,961	$ 169,673	503,694	648,884	$ 499,925
Adjustments to reconcile net income to net cash from operating activities:
Net investment gains									(92,324)	(254,852)	(121,544)
Depreciation and amortization									85,139	88,836	103,090
Stock incentive plans									32,123	28,068	23,784
Change in:
Other liabilities									(63,680)	(58,254)	42,787
Net cash from operating activities									881,304	734,847	819,798
Cash from (used in) investing activities:
Proceeds from sale of fixed maturity securities									1,388,680	633,459	1,344,707
Proceeds from maturities and prepayments of fixed maturity securities									2,999,339	2,605,839	2,718,156
Proceeds from sales of equity securities									15,833	113,251	267,554
Cost of purchases of fixed maturity securities									(4,455,223)	(4,292,165)	(4,198,135)
Payments to Acquire Available-for-sale Securities, Equity									(29,526)	(31,207)	(156,557)
Payments to Acquire Businesses, Net of Cash Acquired									(7,312)	(65,421)	(56,878)
Change in balance due to security broker									(22,666)	32,617	(26,155)
Net additions to real estate, furniture & equipment									(63,562)	(41,958)	(63,150)
Net cash used in investing activities									(170,763)	(808,311)	(545,364)
Cash from (used in) financing activities:
Net proceeds from stock options exercised									0	0	53
Purchase of common treasury shares									(223,652)	(238,933)	(166,473)
Cash dividends to common stockholders									(58,034)	(181,489)	(52,717)
Net cash used in financing activities									(555,318)	(69,773)	(330,262)
Net increase (decrease) in cash and cash equivalents									89,190	(165,297)	(65,932)
Cash and cash equivalents at beginning of year				674,441				839,738	674,441	839,738	905,670
Cash and cash equivalents at end of year	763,631				674,441				763,631	674,441	839,738
Parent Company
Condensed Cash Flow Statements, Captions [Line Items]
Fixed maturity securities available for sale at fair value (cost $201,256 and $272,283 at December 31, 2015 and 2014, respectively)	201,738				273,773				201,738	273,773
Cash flows from operating activities:
Net income									503,694	648,884	499,925
Adjustments to reconcile net income to net cash from operating activities:
Net investment gains									(696)	(5,487)	(24,550)
Depreciation and amortization									2,693	2,916	10,397
Equity in undistributed earnings of subsidiaries									(46,113)	(307,401)	(369,640)
Tax payments received from subsidiaries									311,482	462,809	77,305
Federal income taxes provided by subsidiaries on a separate return basis									(272,180)	(366,721)	(225,845)
Stock incentive plans									29,725	28,068	23,784
Change in:
Federal income taxes									51,772	(15,239)	21,866
Other assets									301	(364)	(821)
Other liabilities									(92,752)	(39,780)	(15,470)
Accrued investment income									524	(820)	967
Net cash from operating activities									488,450	406,865	(2,082)
Cash from (used in) investing activities:
Proceeds from sale of fixed maturity securities									380,986	289,683	230,854
Proceeds from maturities and prepayments of fixed maturity securities									123,639	103,646	68,918
Proceeds from sales of equity securities									308	7,356	23,395
Cost of purchases of fixed maturity securities									(432,645)	(605,768)	(79,132)
Payments to Acquire Available-for-sale Securities, Equity									0	0	(4,668)
Payments to Acquire Businesses, Net of Cash Acquired									0	(82,879)	0
Investments in and advances to subsidiaries, net									30,338	34,191	(58,454)
Change in balance due to security broker									0	(2,151)	6,918
Net additions to real estate, furniture & equipment									(4,425)	(1,615)	(1,896)
Net cash used in investing activities									98,201	(257,537)	185,935
Cash from (used in) financing activities:
Net proceeds from issuance of senior notes									0	344,472	339,627
Net proceeds from stock options exercised									0	0	53
Repayment of senior notes									(200,000)	0	(450,000)
Purchase of common treasury shares									(223,652)	(238,933)	(166,473)
Cash dividends to common stockholders									(58,034)	(181,489)	(52,717)
Net cash used in financing activities									(481,686)	(75,950)	(329,510)
Net increase (decrease) in cash and cash equivalents									104,965	73,378	(145,657)
Cash and cash equivalents at beginning of year				$ 90,693				$ 17,315	90,693	17,315
Cash and cash equivalents at end of year	$ 195,658				$ 90,693				$ 195,658	$ 90,693	$ 17,315