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	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	$ 283,674	$ 2,610,752
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(5,741)	(56,485)
Available-for-sale securities, estimated fair value, 12 months or longer	2,032,512	1,983,912
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(47,516)	(77,504)
Available-for-sale securities, estimated fair value	2,316,186	4,594,664
Available-for-sale securities, gross unrealized holding losses	(53,257)	(133,989)
U.S. Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	0	2,922
Available-for-sale securities, gross unrealized holding losses, less than 12 months	0	(83)
Available-for-sale securities, estimated fair value, 12 months or longer	127,227	156,352
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(1,113)	(1,869)
Available-for-sale securities, estimated fair value	127,227	159,274
Available-for-sale securities, gross unrealized holding losses	(1,113)	(1,952)
U.S. Government-Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	0	88,854
Available-for-sale securities, gross unrealized holding losses, less than 12 months	0	(1,923)
Available-for-sale securities, estimated fair value, 12 months or longer	93,014	96,337
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(843)	(2,191)
Available-for-sale securities, estimated fair value	93,014	185,191
Available-for-sale securities, gross unrealized holding losses	(843)	(4,114)
Obligations Of States, Municipalities And Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	4,781	656,573
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(33)	(12,455)
Available-for-sale securities, estimated fair value, 12 months or longer	370,149	453,736
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(6,158)	(16,413)
Available-for-sale securities, estimated fair value	374,930	1,110,309
Available-for-sale securities, gross unrealized holding losses	(6,191)	(28,868)
Foreign Governments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	137,317	419,764
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(3,048)	(14,461)
Available-for-sale securities, estimated fair value, 12 months or longer	147,720	84,776
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(5,645)	(7,117)
Available-for-sale securities, estimated fair value	285,037	504,540
Available-for-sale securities, gross unrealized holding losses	(8,693)	(21,578)
Commercial Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	8,865	653,410
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(157)	(10,128)
Available-for-sale securities, estimated fair value, 12 months or longer	724,587	709,971
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(23,859)	(34,399)
Available-for-sale securities, estimated fair value	733,452	1,363,381
Available-for-sale securities, gross unrealized holding losses	(24,016)	(44,527)
Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	1,874	276,777
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(12)	(3,685)
Available-for-sale securities, estimated fair value, 12 months or longer	245,331	242,949
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(4,532)	(8,814)
Available-for-sale securities, estimated fair value	247,205	519,726
Available-for-sale securities, gross unrealized holding losses	(4,544)	(12,499)
Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	0	1,645
Available-for-sale securities, gross unrealized holding losses, less than 12 months	0	(11)
Available-for-sale securities, estimated fair value, 12 months or longer	12,371	17,030
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(105)	(202)
Available-for-sale securities, estimated fair value	12,371	18,675
Available-for-sale securities, gross unrealized holding losses	(105)	(213)
Corporate Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	56,587	313,164
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(1,385)	(10,965)
Available-for-sale securities, estimated fair value, 12 months or longer	312,113	222,761
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(5,261)	(6,499)
Available-for-sale securities, estimated fair value	368,700	535,925
Available-for-sale securities, gross unrealized holding losses	(6,646)	(17,464)
Total Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	209,424	2,413,109
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(4,635)	(53,711)
Available-for-sale securities, estimated fair value, 12 months or longer	2,032,512	1,983,912
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(47,516)	(77,504)
Available-for-sale securities, estimated fair value	2,241,936	4,397,021
Available-for-sale securities, gross unrealized holding losses	(52,151)	(131,215)
Short-Term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	74,250	197,643
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(1,106)	(2,774)
Available-for-sale securities, estimated fair value, 12 months or longer	0	0
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	0	0
Available-for-sale securities, estimated fair value	74,250	197,643
Available-for-sale securities, gross unrealized holding losses	$ (1,106)	$ (2,774)