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	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	$ 1,978,272	$ 3,469,062
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(39,065)	(82,777)
Available-for-sale securities, estimated fair value, 12 months or longer	348,527	321,552
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(12,773)	(17,204)
Available-for-sale securities, estimated fair value	2,326,799	3,790,614
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(51,838)	(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	102,240	122,950
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(585)	(894)
Available-for-sale securities, estimated fair value, 12 months or longer	23,609	0
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(274)	0
Available-for-sale securities, estimated fair value	125,849	122,950
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(859)	(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	102,957	220,333
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(1,237)	(4,324)
Available-for-sale securities, estimated fair value, 12 months or longer	1,744	7,618
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(3)	(4)
Available-for-sale securities, estimated fair value	104,701	227,951
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(1,240)	(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	525,844	1,004,947
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(6,801)	(37,685)
Available-for-sale securities, estimated fair value, 12 months or longer	143,119	31,723
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(5,988)	(4,120)
Available-for-sale securities, estimated fair value	668,963	1,036,670
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(12,789)	(41,805)
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	135,018	68,887
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(3,594)	(2,145)
Available-for-sale securities, estimated fair value, 12 months or longer	7,158	5,005
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(7)	(8)
Available-for-sale securities, estimated fair value	142,176	73,892
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(3,601)	(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	569,763	749,889
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(12,071)	(19,091)
Available-for-sale securities, estimated fair value, 12 months or longer	13,486	29,988
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(256)	(453)
Available-for-sale securities, estimated fair value	583,249	779,877
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(12,327)	(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	106,673	181,557
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(1,501)	(4,987)
Available-for-sale securities, estimated fair value, 12 months or longer	70,723	79,936
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(1,578)	(2,319)
Available-for-sale securities, estimated fair value	177,396	261,493
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(3,079)	(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	9,676	14,501
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(38)	(106)
Available-for-sale securities, estimated fair value, 12 months or longer	6,561	5,869
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(34)	(52)
Available-for-sale securities, estimated fair value	16,237	20,370
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(72)	(158)
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	266,040	494,573
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(2,275)	(8,357)
Available-for-sale securities, estimated fair value, 12 months or longer	69,916	93,790
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(704)	(1,680)
Available-for-sale securities, estimated fair value	335,956	588,363
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(2,979)	(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	1,818,211	2,857,637
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(28,102)	(77,589)
Available-for-sale securities, estimated fair value, 12 months or longer	336,316	253,929
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(8,844)	(8,636)
Available-for-sale securities, estimated fair value	2,154,527	3,111,566
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(36,946)	(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	23,636	8,808
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(2,616)	(410)
Available-for-sale securities, estimated fair value, 12 months or longer	1,099	37,973
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(802)	(5,186)
Available-for-sale securities, estimated fair value	24,735	46,781
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(3,418)	(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	60,596	98,406
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(8,333)	(4,772)
Available-for-sale securities, estimated fair value, 12 months or longer	11,112	29,650
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(3,127)	(3,382)
Available-for-sale securities, estimated fair value	71,708	128,056
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(11,460)	(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	84,232	107,214
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(10,949)	(5,182)
Available-for-sale securities, estimated fair value, 12 months or longer	12,211	67,623
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(3,929)	(8,568)
Available-for-sale securities, estimated fair value	96,443	174,837
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(14,878)	(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	75,829	504,211
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	(14)	(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	75,829	504,211
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	$ (14)	$ (6)