Investments (Available-For-Sale Investments) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	$ 6,937,684	$ 6,870,260
Available-for-sale investments, gross unrealized holding gains	1,430,691	1,100,525
Available-for-sale investments, gross unrealized holding losses	(8,482)	(8,798)
Available-for-sale investments, unrealized other-than-temporary impairment losses	(9,049)	(8,872)
Available-for-sale investments, estimated fair value	8,350,844	7,953,115
U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	300,491	299,413
Available-for-sale investments, gross unrealized holding gains	21,462	22,789
Available-for-sale investments, gross unrealized holding losses	0	(9)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	321,953	322,193
Obligations Of States, Municipalities And Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	2,653,298	2,729,838
Available-for-sale investments, gross unrealized holding gains	257,461	201,477
Available-for-sale investments, gross unrealized holding losses	(341)	(794)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	2,910,418	2,930,521
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	544,544	572,253
Available-for-sale investments, gross unrealized holding gains	57,673	45,629
Available-for-sale investments, gross unrealized holding losses	(1)	(1,068)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	602,216	616,814
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	261,089	366,859
Available-for-sale investments, gross unrealized holding gains	19,515	24,601
Available-for-sale investments, gross unrealized holding losses	(3)	(18)
Available-for-sale investments, unrealized other-than-temporary impairment losses	(2,258)	(2,258)
Available-for-sale investments, estimated fair value	278,343	389,184
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	14,332	16,096
Available-for-sale investments, gross unrealized holding gains	676	731
Available-for-sale investments, gross unrealized holding losses	0	(9)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	15,008	16,818
Public Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	56,432	63,965
Available-for-sale investments, gross unrealized holding gains	5,221	5,462
Available-for-sale investments, gross unrealized holding losses	0	0
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	61,653	69,427
All Other Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,003,063	1,124,528
Available-for-sale investments, gross unrealized holding gains	94,765	78,053
Available-for-sale investments, gross unrealized holding losses	(79)	(2,750)
Available-for-sale investments, unrealized other-than-temporary impairment losses	(6,791)	(6,614)
Available-for-sale investments, estimated fair value	1,090,958	1,193,217
Total Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	4,833,249	5,172,952
Available-for-sale investments, gross unrealized holding gains	456,773	378,742
Available-for-sale investments, gross unrealized holding losses	(424)	(4,648)
Available-for-sale investments, unrealized other-than-temporary impairment losses	(9,049)	(8,872)
Available-for-sale investments, estimated fair value	5,280,549	5,538,174
Insurance Companies, Banks And Trusts [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	442,119	389,421
Available-for-sale investments, gross unrealized holding gains	383,397	296,648
Available-for-sale investments, gross unrealized holding losses	(1,798)	(1,366)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	823,718	684,703
Industrial, Consumer And All Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	933,287	766,873
Available-for-sale investments, gross unrealized holding gains	590,506	425,131
Available-for-sale investments, gross unrealized holding losses	(6,258)	(2,780)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,517,535	1,189,224
Total Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,375,406	1,156,294
Available-for-sale investments, gross unrealized holding gains	973,903	721,779
Available-for-sale investments, gross unrealized holding losses	(8,056)	(4,146)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	2,341,253	1,873,927
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	729,029	541,014
Available-for-sale investments, gross unrealized holding gains	15	4
Available-for-sale investments, gross unrealized holding losses	(2)	(4)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	$ 729,042	$ 541,014