Revisions To Financial Statements (Schedule of Prior Period Financial Statments) (Details) - USD ($) $ in Thousands	3 Months Ended					6 Months Ended		9 Months Ended	12 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred acquisition costs												$ 48,827	$ 57,314	$ 68,621	$ 86,402
TOTAL ASSETS												17,761,659	17,582,846	17,083,436	16,688,954
Other net payable to reinsurers												906,305	790,561	815,298	1,187,780
Income taxes												173,693	229,254	148,940	72,156
Total liabilities												12,238,865	12,103,502	11,784,883	11,730,644
Retained earnings												5,152,137	5,116,479	4,947,301	4,645,657
Total stockholder's equity	$ 5,479,344	$ 5,298,553	$ 5,194,238	$ 5,065,880	$ 4,958,310	$ 5,298,553	$ 4,958,310	$ 4,958,310	$ 4,958,310	$ 4,600,079	$ 4,600,079	5,522,794	5,479,344	5,298,553	4,958,310
TOTAL LIABILITIES AND STOCKHOLDER'S EQUITY												17,761,659	17,582,846	17,083,436	16,688,954
Commission, brokerage, taxes and fees	60,513	52,507	82,778	70,409	66,365	113,020	136,774	219,552	281,424	312,325	338,975
Total claims and expenses	431,404	414,580	459,224	478,143	432,849	845,984	910,992	1,370,216	1,920,382	1,889,318	1,930,322
INCOME (LOSS) BEFORE TAXES	27,057	244,947	98,728	103,049	54,246	272,004	157,295	256,023	398,991	607,286	658,115
Income tax expense (benefit)	(8,601)	75,769	22,425	33,677	13,256	67,168	46,933	69,358	97,347	194,896	204,687
Net income (loss)	35,658	169,178	76,303	69,372	40,990	204,836	110,362	186,665	301,644	412,390	453,428
COMPREHENSIVE INCOME (LOSS)	43,382	180,721	75,970	125,403	102,777	224,103	228,180	304,150	327,465	345,735	370,299
Balance, beginning of period	5,479,344	5,298,553	5,194,238	5,065,880	4,958,310	5,298,553	4,958,310	4,958,310	4,958,310	4,600,079
Balance, end of period	5,522,794	5,479,344	5,272,645	5,194,238	5,065,880	5,522,794	5,194,238	5,272,645	5,298,553	4,958,310	4,600,079
Decrease (increase) in income taxes		74,132			6,939	14,411	9,279	35,647	62,991	60,494	69,331
Increase (decrease) in other net payable to reinsurers		(27,178)			(107,710)	89,702	(373,353)	(216,024)	(371,856)	195,936	1,914
Change in other assets and liabilities, net		17,895			23,161	(10,031)	13,152	(125,150)	15,144	13,345	84,177
As Previously Reported [Member]
Deferred acquisition costs													62,308	73,924	92,651
TOTAL ASSETS													17,587,840	17,088,739	16,695,203
Other net payable to reinsurers													832,307	860,391	1,225,260
Income taxes													223,629	142,143	68,024
Total liabilities													12,139,623	11,823,179	11,763,992
Retained earnings													5,085,352	4,914,308	4,618,558
Total stockholder's equity	5,448,217	5,265,560	5,164,053	5,036,717	4,931,211	5,265,560	4,931,211	4,931,211	4,931,211	4,572,717	4,572,717		5,448,217	5,265,560	4,931,211
TOTAL LIABILITIES AND STOCKHOLDER'S EQUITY													17,587,840	17,088,739	16,695,203
Commission, brokerage, taxes and fees		49,470	85,563	72,126	68,822		140,948	226,511	289,982	315,069	339,402
Total claims and expenses		411,543	462,009	479,860	435,306		915,166	1,377,175	1,928,940	1,892,062	1,930,749
INCOME (LOSS) BEFORE TAXES		247,984	95,943	101,332	51,789		153,121	249,064	390,433	604,542	657,688
Income tax expense (benefit)		76,940	21,145	32,982	12,863		45,845	66,990	94,683	191,889	203,562
Net income (loss)		171,044	74,798	68,350	38,926		107,276	182,074	295,750	412,653	454,126
COMPREHENSIVE INCOME (LOSS)		182,587	74,465	124,381	100,713		225,094	299,559	321,571	345,998	370,997
Balance, beginning of period	5,448,217	5,265,560	5,164,053	5,036,717	4,931,211	5,265,560	4,931,211	4,931,211	4,931,211	4,572,717
Balance, end of period		5,448,217	5,240,955	5,164,053	5,036,717		5,164,053	5,240,955	5,265,560	4,931,211	4,572,717
Decrease (increase) in income taxes		75,304			6,546		8,190	33,279	60,325	57,487	68,206
Increase (decrease) in other net payable to reinsurers		(30,525)			(106,588)		(370,242)	(209,260)	(364,242)	204,526	5,130
Change in other assets and liabilities, net		18,204			24,496		14,216	(124,955)	16,090	7,499	81,388
Impact Of Revisions [Member]
Deferred acquisition costs													(4,994)	(5,303)	(6,249)
TOTAL ASSETS													(4,994)	(5,303)	(6,249)
Other net payable to reinsurers													(41,746)	(45,093)	(37,480)
Income taxes													5,625	6,797	4,132
Total liabilities													(36,121)	(38,296)	(33,348)
Retained earnings													31,127	32,993	27,099
Total stockholder's equity	31,127	32,993	30,185	29,163	27,099	32,993	27,099	27,099	27,099	27,362	27,362		31,127	32,993	27,099
TOTAL LIABILITIES AND STOCKHOLDER'S EQUITY													(4,994)	(5,303)	(6,249)
Commission, brokerage, taxes and fees		3,037	(2,785)	(1,717)	(2,457)		(4,174)	(6,959)	(8,558)	(2,744)	(427)
Total claims and expenses		3,037	(2,785)	(1,717)	(2,457)		(4,174)	(6,959)	(8,558)	(2,744)	(427)
INCOME (LOSS) BEFORE TAXES		(3,037)	2,785	1,717	2,457		4,174	6,959	8,558	2,744	427
Income tax expense (benefit)		(1,171)	1,280	695	393		1,088	2,368	2,664	3,007	1,125
Net income (loss)		(1,866)	1,505	1,022	2,064		3,086	4,591	5,894	(263)	(698)
COMPREHENSIVE INCOME (LOSS)		(1,866)	1,505	1,022	2,064		3,086	4,591	5,894	(263)	(698)
Balance, beginning of period	31,127	32,993	30,185	29,163	27,099	32,993	27,099	27,099	27,099	27,362
Balance, end of period		31,127	31,690	30,185	29,163		30,185	31,690	32,993	27,099	27,362
Decrease (increase) in income taxes		(1,172)			393		1,089	2,368	2,666	3,007	1,125
Increase (decrease) in other net payable to reinsurers		3,347			(1,122)		(3,111)	(6,764)	(7,614)	(8,590)	(3,216)
Change in other assets and liabilities, net		(309)			(1,335)		(1,064)	(195)	(946)	5,846	2,789
Retained Earnings [Member]
Total stockholder's equity	5,116,479	4,947,301	4,756,019	4,686,647	4,645,657	4,947,301	4,645,657	4,645,657	4,645,657	4,233,267	3,779,839	$ 5,152,137	5,116,479	4,947,301	4,645,657
Net income (loss)	35,658	169,178	76,303	69,372	40,990	204,836	110,362	186,665	301,644	412,390	453,428
Balance, beginning of period	5,116,479	4,947,301	4,756,019	4,686,647	4,645,657	4,947,301	4,645,657	4,645,657	4,645,657	4,233,267	3,779,839
Balance, end of period	5,152,137	5,116,479	4,832,322	4,756,019	4,686,647	5,152,137	4,756,019	4,832,322	4,947,301	4,645,657	4,233,267
Retained Earnings [Member] | As Previously Reported [Member]
Total stockholder's equity	5,085,352	4,914,308	4,725,834	4,657,484	4,618,558	4,914,308	4,618,558	4,618,558	4,618,558	4,205,905	3,751,779		5,085,352	4,914,308	4,618,558
Net income (loss)		171,044	74,798	68,350	38,926		107,276	182,074	295,750	412,653	454,126
Balance, beginning of period	5,085,352	4,914,308	4,725,834	4,657,484	4,618,558	4,914,308	4,618,558	4,618,558	4,618,558	4,205,905	3,751,779
Balance, end of period		5,085,352	4,800,632	4,725,834	4,657,484		4,725,834	4,800,632	4,914,308	4,618,558	4,205,905
Retained Earnings [Member] | Impact Of Revisions [Member]
Total stockholder's equity	31,127	32,993	30,185	29,163	27,099	32,993	27,099	27,099	27,099	27,362	28,060		$ 31,127	$ 32,993	$ 27,099
Net income (loss)		(1,866)	1,505	1,022	2,064		3,086	4,591	5,894	(263)	(698)
Balance, beginning of period	$ 31,127	32,993	30,185	29,163	27,099	$ 32,993	27,099	27,099	27,099	27,362	28,060
Balance, end of period		$ 31,127	$ 31,690	$ 30,185	$ 29,163		$ 30,185	$ 31,690	$ 32,993	$ 27,099	$ 27,362