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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 119,306	$ 165,208	$ 216,709	$ 180,722	$ 132,357	$ 161,920	$ 180,075	$ 166,397	$ 681,944	$ 640,749	$ 549,094
Adjustments to reconcile net income to net cash from operating activities:
Depreciation and amortization									113,387	131,108	112,956
Stock incentive plans									49,274	36,591	40,490
Change in:
Other liabilities									(19,506)	(48,770)	66,482
Net cash from operating activities									1,143,793	620,199	710,883
Cash from (used) in investing activities:
Proceeds from sale of fixed maturity securities									2,093,271	3,525,149	4,035,162
Proceeds from maturities and prepayments of fixed maturity securities									2,933,980	2,676,455	3,556,744
Cost of purchases of fixed maturity securities									(5,352,886)	(6,677,753)	(7,940,957)
Change in loans receivable									3,481	(13,204)	27,135
Change in balances due from security brokers									2,844	4,262	(4,372)
Net additions to real estate, furniture & equipment									(60,457)	(49,860)	(115,719)
Other, net									(21,391)	(17,740)	(6,043)
Net cash used in investing activities									(424,871)	(714,244)	(333,464)
Cash used in financing activities:
Purchase of common treasury shares									(18,225)	(24,750)	(47,807)
Cash dividends to common stockholders									(308,191)	(254,951)	(188,199)
Net cash used in financing activities									(513,193)	(7,403)	(235,086)
Net increase (decrease) in cash and cash equivalents									206,108	(132,869)	155,186
Cash and cash equivalents at beginning of year				817,602				950,471	817,602	950,471	795,285
Cash and cash equivalents at end of year	1,023,710				817,602				1,023,710	817,602	950,471
Parent Company
Cash flows from operating activities:
Net income									681,944	640,749	549,094
Adjustments to reconcile net income to net cash from operating activities:
Net investment (gains) losses									(850)	1,685	4,286
Depreciation and amortization									7,058	9,441	2,039
Equity in undistributed earnings of subsidiaries									(496,841)	5,129	(222,390)
Tax payments received from subsidiaries									192,407	282,084	98,313
Federal income taxes provided by subsidiaries on a separate return basis									(207,646)	(409,439)	(115,597)
Stock incentive plans									28,389	28,531	38,075
Change in:
Federal income taxes									11,841	(77,415)	2,711
Other assets									(5,343)	1,348	(877)
Other liabilities									11,866	109,016	18,661
Accrued investment income									4,395	(2,870)	(2,818)
Net cash from operating activities									227,220	588,259	371,497
Cash from (used) in investing activities:
Proceeds from sale of fixed maturity securities									619,334	668,447	849,330
Proceeds from maturities and prepayments of fixed maturity securities									435,473	255,528	316,611
Cost of purchases of fixed maturity securities									(459,418)	(1,188,821)	(1,329,379)
Change in loans receivable									(4,250)	1,475	(29,600)
Investments in and advances to subsidiaries, net									(36,170)	(184,597)	(21,139)
Change in balances due from security brokers									245	0	0
Net additions to real estate, furniture & equipment									(112)	(264)	(1,055)
Other, net									142	0	0
Net cash used in investing activities									555,244	(448,232)	(215,232)
Cash used in financing activities:
Net proceeds from issuance of senior notes									290,454	178,562	0
Repayment of senior notes									(440,651)	0	0
Purchase of common treasury shares									(18,225)	(24,750)	(47,807)
Cash dividends to common stockholders									(308,191)	(254,951)	(188,199)
Net cash used in financing activities									(476,613)	(101,139)	(236,006)
Net increase (decrease) in cash and cash equivalents									305,851	38,888	(79,741)
Cash and cash equivalents at beginning of year				$ 83,950				$ 45,062	83,950	45,062
Cash and cash equivalents at end of year	$ 389,801				$ 83,950				$ 389,801	$ 83,950	$ 45,062