Schedule II - Condensed Financial Information of Registrant (Statements of Cash Flows (Parent Company)) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Cash Flow Statements, Captions [Line Items]
Fixed maturity securities available for sale at fair value (cost $899,206 and $201,256 at December 31, 2016 and 2015, respectively)	$ 13,190,668		$ 12,444,394						$ 13,190,668	$ 12,444,394
Cash flows from operating activities:
Net income	152,790	$ 119,511	109,745	$ 118,307	$ 108,967	$ 123,035	$ 220,650	$ 152,607	601,916	503,694	$ 648,884
Adjustments to reconcile net income to net cash from operating activities:
Net investment gains									(267,005)	(92,324)	(254,852)
Depreciation and amortization									86,051	85,139	88,836
Equity in undistributed earnings of subsidiaries									(103,944)
Stock incentive plans									37,174	32,123	28,068
Change in:
Other liabilities									46,852	(63,680)	(58,254)
Net cash from operating activities									848,376	881,304	734,847
Cash from (used in) investing activities:
Proceeds from sale of fixed maturity securities									2,440,310	1,388,680	633,459
Proceeds from maturities and prepayments of fixed maturity securities									2,189,365	2,999,339	2,605,839
Proceeds from sales of equity securities									143,042	15,833	113,251
Cost of purchases of fixed maturity securities									(5,541,202)	(4,455,223)	(4,292,165)
Payments to Acquire Available-for-sale Securities, Equity									(202,736)	(29,526)	(31,207)
Payments to Acquire Businesses, Net of Cash Acquired									(53,451)	(7,312)	(65,421)
Change in balance due to security broker									20,992	(22,666)	32,617
Net additions to real estate, furniture & equipment									(50,829)	(63,562)	(41,958)
Net cash used in investing activities									(794,488)	(170,763)	(808,311)
Cash from (used in) financing activities:
Purchase of common treasury shares									(132,392)	(223,652)	(238,933)
Cash dividends to common stockholders									(183,999)	(58,034)	(181,489)
Net cash used in financing activities									(6,932)	(555,318)	(69,773)
Net increase (decrease) in cash and cash equivalents									31,654	89,190	(165,297)
Cash and cash equivalents at beginning of year		763,631		674,441	763,631	674,441	763,631	674,441	763,631	674,441
Cash and cash equivalents at end of year	795,285		763,631						795,285	763,631	674,441
Parent Company
Condensed Cash Flow Statements, Captions [Line Items]
Fixed maturity securities available for sale at fair value (cost $899,206 and $201,256 at December 31, 2016 and 2015, respectively)	894,748		201,738						894,748	201,738
Cash flows from operating activities:
Net income	601,916								601,919	503,694	648,884
Adjustments to reconcile net income to net cash from operating activities:
Net investment gains									3,649	(696)	(5,487)
Depreciation and amortization									2,744	2,693	2,916
Equity in undistributed earnings of subsidiaries									(103,941)	(46,113)	(307,401)
Tax payments received from subsidiaries									414,386	311,482	462,809
Federal income taxes provided by subsidiaries on a separate return basis									(327,520)	(272,180)	(366,721)
Stock incentive plans									37,174	29,725	28,068
Change in:
Federal income taxes									44,839	51,772	(15,239)
Other assets									1,772	301	(364)
Other liabilities									(88,282)	(92,752)	(39,780)
Accrued investment income									(2,743)	524	(820)
Net cash from operating activities									583,994	488,450	406,865
Cash from (used in) investing activities:
Proceeds from sale of fixed maturity securities									373,252	380,986	289,683
Proceeds from maturities and prepayments of fixed maturity securities									210,904	123,639	103,646
Proceeds from sales of equity securities									0	308	7,356
Cost of purchases of fixed maturity securities									(1,285,101)	(432,645)	(605,768)
Payments to Acquire Businesses, Net of Cash Acquired									0	0	(82,879)
Investments in and advances to subsidiaries, net									11,471	30,338	34,191
Change in balance due to security broker									0	0	(2,151)
Net additions to real estate, furniture & equipment									(3,042)	(4,425)	(1,615)
Net cash used in investing activities									(715,935)	98,201	(257,537)
Cash from (used in) financing activities:
Net proceeds from issuance of senior notes									386,830	0	344,472
Repayment of senior notes									(9,353)	(200,000)	0
Purchase of common treasury shares									(132,392)	(223,652)	(238,933)
Cash dividends to common stockholders									(183,999)	(58,034)	(181,489)
Net cash used in financing activities									61,086	(481,686)	(75,950)
Net increase (decrease) in cash and cash equivalents									(70,855)	104,965	73,378
Cash and cash equivalents at beginning of year		$ 195,658		$ 90,693	$ 195,658	$ 90,693	$ 195,658	$ 90,693	195,658	90,693
Cash and cash equivalents at end of year	$ 124,803		$ 195,658						$ 124,803	$ 195,658	$ 90,693