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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ 132,357	$ 161,920	$ 180,075	$ 166,397	$ 154,589	$ 162,054	$ 109,004	$ 123,447	$ 640,749	$ 549,094	$ 601,916
Adjustments to reconcile net income to net cash from operating activities:
Depreciation and amortization									131,108	112,956	86,051
Stock incentive plans									36,591	40,490	37,174
Change in:
Other liabilities									(48,770)	66,482	46,852
Net cash from operating activities									620,199	710,883	848,376
Cash used in investing activities:
Proceeds from sale of fixed maturity securities									3,525,149	4,035,162	2,440,310
Proceeds from maturities and prepayments of fixed maturity securities									2,676,455	3,556,744	2,189,365
Proceeds from sales of equity securities									497,989	195,270	143,042
Cost of purchases of fixed maturity securities									(6,677,753)	(7,940,957)	(5,541,202)
Change in loans receivable									(13,204)	27,135	166,327
Net additions to real estate, furniture & equipment									(49,860)	(115,719)	(50,829)
Net cash used in investing activities									(714,244)	(333,464)	(794,488)
Cash (used in) from financing activities:
Purchase of common treasury shares									(24,750)	(47,807)	(132,392)
Cash dividends to common stockholders									(254,951)	(188,199)	(183,999)
Net cash used in financing activities									(7,403)	(235,086)	(6,932)
Net (decrease) increase in cash and cash equivalents									(132,869)	155,186	31,654
Cash and cash equivalents at beginning of year				950,471				795,285	950,471	795,285	763,631
Cash and cash equivalents at end of year	817,602				950,471				817,602	950,471	795,285
Parent Company
Cash flows from operating activities:
Net income									640,749	549,094	601,916
Adjustments to reconcile net income to net cash from operating activities:
Net investment losses									1,685	4,286	3,649
Depreciation and amortization									9,441	2,039	2,744
Equity in undistributed earnings of subsidiaries									5,129	(222,390)	(103,941)
Tax payments received from subsidiaries									282,084	98,313	414,386
Federal income taxes provided by subsidiaries on a separate return basis									(409,439)	(115,597)	(327,520)
Stock incentive plans									28,531	38,075	37,174
Change in:
Federal income taxes									(77,415)	2,711	44,839
Other assets									1,348	(877)	1,772
Other liabilities									109,016	18,661	(88,282)
Accrued investment income									(2,870)	(2,818)	(2,743)
Net cash from operating activities									588,259	371,497	583,994
Cash used in investing activities:
Proceeds from sale of fixed maturity securities									668,447	849,330	373,252
Proceeds from maturities and prepayments of fixed maturity securities									255,528	316,611	210,904
Cost of purchases of fixed maturity securities									(1,188,821)	(1,329,379)	(1,285,101)
Change in loans receivable									1,475	(29,600)	(23,419)
Investments in and advances to subsidiaries, net									(184,597)	(21,139)	11,471
Net additions to real estate, furniture & equipment									(264)	(1,055)	(3,042)
Net cash used in investing activities									(448,232)	(215,232)	(715,935)
Cash (used in) from financing activities:
Net proceeds from issuance of senior notes									178,562	0	386,830
Repayment of senior notes									0	0	(9,353)
Purchase of common treasury shares									(24,750)	(47,807)	(132,392)
Cash dividends to common stockholders									(254,951)	(188,199)	(183,999)
Net cash used in financing activities									(101,139)	(236,006)	61,086
Net (decrease) increase in cash and cash equivalents									38,888	(79,741)	(70,855)
Cash and cash equivalents at beginning of year				$ 45,062				$ 124,803	45,062	124,803
Cash and cash equivalents at end of year	$ 83,950				$ 45,062				$ 83,950	$ 45,062	$ 124,803