Investments (Summary Of Gross Unrealized Investment Losses By Length Of Time That Securities Have Continuously Been In An Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	$ 2,270,411	$ 3,469,062
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(44,928)	(82,777)
Available-for-sale securities, estimated fair value, 12 months or longer	218,258	321,552
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(9,747)	(17,204)
Available-for-sale securities, estimated fair value	2,488,669	3,790,614
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(54,675)	(99,981)
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	95,984	122,950
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(741)	(894)
Available-for-sale securities, estimated fair value, 12 months or longer	7,427	0
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(103)	0
Available-for-sale securities, estimated fair value	103,411	122,950
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(844)	(894)
U.S. Government-Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	137,702	220,333
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(1,426)	(4,324)
Available-for-sale securities, estimated fair value, 12 months or longer	0	7,618
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	(4)
Available-for-sale securities, estimated fair value	137,702	227,951
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(1,426)	(4,328)
Obligations Of States, Municipalities And Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	706,155	1,004,947
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(17,574)	(37,685)
Available-for-sale securities, estimated fair value, 12 months or longer	31,626	31,723
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(3,058)	(4,120)
Available-for-sale securities, estimated fair value	737,781	1,036,670
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(20,632)	(41,805)
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	122,855	68,887
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(2,231)	(2,145)
Available-for-sale securities, estimated fair value, 12 months or longer	0	5,005
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	(8)
Available-for-sale securities, estimated fair value	122,855	73,892
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(2,231)	(2,153)
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	536,397	749,889
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(12,801)	(19,091)
Available-for-sale securities, estimated fair value, 12 months or longer	14,693	29,988
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(253)	(453)
Available-for-sale securities, estimated fair value	551,090	779,877
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(13,054)	(19,544)
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	123,791	181,557
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(1,806)	(4,987)
Available-for-sale securities, estimated fair value, 12 months or longer	74,672	79,936
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(1,884)	(2,319)
Available-for-sale securities, estimated fair value	198,463	261,493
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(3,690)	(7,306)
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	22,992	14,501
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(51)	(106)
Available-for-sale securities, estimated fair value, 12 months or longer	5,106	5,869
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(33)	(52)
Available-for-sale securities, estimated fair value	28,098	20,370
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(84)	(158)
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	378,552	494,573
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(2,351)	(8,357)
Available-for-sale securities, estimated fair value, 12 months or longer	74,071	93,790
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(950)	(1,680)
Available-for-sale securities, estimated fair value	452,623	588,363
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(3,301)	(10,037)
Total Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	2,124,428	2,857,637
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(38,981)	(77,589)
Available-for-sale securities, estimated fair value, 12 months or longer	207,595	253,929
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(6,281)	(8,636)
Available-for-sale securities, estimated fair value	2,332,023	3,111,566
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(45,262)	(86,225)
Insurance, Banks And Other Financial Institutions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	955	8,808
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(60)	(410)
Available-for-sale securities, estimated fair value, 12 months or longer	1,375	37,973
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(526)	(5,186)
Available-for-sale securities, estimated fair value	2,330	46,781
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(586)	(5,596)
Industrial, Consumer And All Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	88,643	98,406
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(5,877)	(4,772)
Available-for-sale securities, estimated fair value, 12 months or longer	9,288	29,650
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(2,940)	(3,382)
Available-for-sale securities, estimated fair value	97,931	128,056
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(8,817)	(8,154)
Total Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	89,598	107,214
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(5,937)	(5,182)
Available-for-sale securities, estimated fair value, 12 months or longer	10,663	67,623
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(3,466)	(8,568)
Available-for-sale securities, estimated fair value	100,261	174,837
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(9,403)	(13,750)
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	56,385	504,211
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(10)	(6)
Available-for-sale securities, estimated fair value, 12 months or longer	0	0
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	0
Available-for-sale securities, estimated fair value	56,385	504,211
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	$ (10)	$ (6)