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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Fixed maturity securities available for sale at fair value (cost $1,059,834 and $899,206 at December 31, 2017 and 2016, respectively)	$ 13,551,250				$ 13,190,668				$ 13,551,250	$ 13,190,668
Cash flows from operating activities:
Net income	154,589	$ 162,054	$ 109,004	$ 123,447	152,790	$ 220,650	$ 108,967	$ 119,511	549,094	601,916	$ 503,694
Adjustments to reconcile net income to net cash from operating activities:
Net investment gains (losses)									(335,858)	(267,005)	(92,324)
Depreciation and amortization									112,956	86,051	85,139
Stock incentive plans									40,490	37,174	32,123
Change in:
Other liabilities									66,482	46,852	(63,680)
Net cash from operating activities									710,883	848,376	881,304
Cash (used in) from investing activities:
Proceeds from sale of fixed maturity securities									4,035,162	2,440,310	1,388,680
Proceeds from maturities and prepayments of fixed maturity securities									3,556,744	2,189,365	2,999,339
Proceeds from sales of equity securities									195,270	143,042	15,833
Cost of purchases of fixed maturity securities									(7,940,957)	(5,541,202)	(4,455,223)
Change in loans receivable									27,135	166,327	48,909
Net additions to real estate, furniture & equipment									(115,719)	(50,829)	(63,562)
Net cash used in investing activities									(333,464)	(794,488)	(170,763)
Cash (used in) from financing activities:
Purchase of common treasury shares									(47,807)	(132,392)	(223,652)
Cash dividends to common stockholders									(188,199)	(183,999)	(58,034)
Net cash used in financing activities									(235,086)	(6,932)	(555,318)
Net increase in cash and cash equivalents									155,186	31,654	89,190
Cash and cash equivalents at beginning of year				795,285				763,631	795,285	763,631	674,441
Cash and cash equivalents at end of year	950,471				795,285				950,471	795,285	763,631
Parent Company
Condensed Cash Flow Statements, Captions [Line Items]
Fixed maturity securities available for sale at fair value (cost $1,059,834 and $899,206 at December 31, 2017 and 2016, respectively)	1,052,240				894,748				1,052,240	894,748
Cash flows from operating activities:
Net income	549,094								549,094	601,916	503,694
Adjustments to reconcile net income to net cash from operating activities:
Net investment gains (losses)									4,286	3,649	(696)
Depreciation and amortization									2,039	2,744	2,693
Equity in undistributed earnings of subsidiaries									(222,390)	(103,941)	(46,113)
Tax payments received from subsidiaries									98,313	414,386	311,482
Federal income taxes provided by subsidiaries on a separate return basis									(115,597)	(327,520)	(272,180)
Stock incentive plans									38,075	37,174	29,725
Change in:
Federal income taxes									2,711	44,839	51,772
Other assets									(877)	1,772	301
Other liabilities									18,661	(88,282)	(92,752)
Accrued investment income									(2,818)	(2,743)	524
Net cash from operating activities									371,497	583,994	488,450
Cash (used in) from investing activities:
Proceeds from sale of fixed maturity securities									849,330	373,252	380,986
Proceeds from maturities and prepayments of fixed maturity securities									316,611	210,904	123,639
Proceeds from sales of equity securities									0	0	308
Cost of purchases of fixed maturity securities									(1,329,379)	(1,285,101)	(432,645)
Change in loans receivable									(29,600)	(23,419)	0
Investments in and advances to subsidiaries, net									(21,139)	11,471	30,338
Net additions to real estate, furniture & equipment									(1,055)	(3,042)	(4,425)
Net cash used in investing activities									(215,232)	(715,935)	98,201
Cash (used in) from financing activities:
Net proceeds from issuance of senior notes									0	386,830	0
Repayment of senior notes									0	(9,353)	(200,000)
Purchase of common treasury shares									(47,807)	(132,392)	(223,652)
Cash dividends to common stockholders									(188,199)	(183,999)	(58,034)
Net cash used in financing activities									(236,006)	61,086	(481,686)
Net increase in cash and cash equivalents									(79,741)	(70,855)	104,965
Cash and cash equivalents at beginning of year				$ 124,803				$ 195,658	124,803	195,658
Cash and cash equivalents at end of year	$ 45,062				$ 124,803				$ 45,062	$ 124,803	$ 195,658