Investments (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	$ 5,240,718	$ 4,864,806
Gross unrealized gain	210,417	170,204
Gross unrealized loss	(16,374)	(35,570)
Fixed income securities - available for sale	5,434,761	4,999,440
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	865,852	1,148,078
Less than 12 months, Unrealized losses	(15,411)	(34,151)
12 months or more, Fair value	20,771	33,859
12 months or more, Unrealized losses	(963)	(1,419)
Total, Fair value	886,623	1,181,937
Total, Unrealized losses	(16,374)	(35,570)
U.S. government and government agency securities [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	353,530	315,339
Gross unrealized gain	9,837	9,097
Gross unrealized loss	(12)	(169)
Fixed income securities - available for sale	363,355	324,267
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	12,816	20,976
Less than 12 months, Unrealized losses	(12)	(169)
12 months or more, Unrealized losses		0
Total, Fair value	12,816	20,976
Total, Unrealized losses	(12)	(169)
Fixed income securities of states, municipalities and political subdivisions [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	997,673	1,050,969
Gross unrealized gain	52,388	38,825
Gross unrealized loss	(2,720)	(7,737)
Fixed income securities - available for sale	1,047,341	1,082,057
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	101,478	228,228
Less than 12 months, Unrealized losses	(2,656)	(7,621)
12 months or more, Fair value	1,585	2,279
12 months or more, Unrealized losses	(64)	(116)
Total, Fair value	103,063	230,507
Total, Unrealized losses	(2,720)	(7,737)
Special purpose revenue bonds of states, municipalities and political subdivisions [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	1,580,332	1,614,554
Gross unrealized gain	55,512	34,764
Gross unrealized loss	(5,464)	(21,259)
Fixed income securities - available for sale	1,630,380	1,628,059
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	313,517	689,190
Less than 12 months, Unrealized losses	(5,225)	(21,156)
12 months or more, Fair value	8,425	6,344
12 months or more, Unrealized losses	(239)	(103)
Total, Fair value	321,942	695,534
Total, Unrealized losses	(5,464)	(21,259)
Corporate fixed income securities [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	681,433	545,883
Gross unrealized gain	27,179	26,436
Gross unrealized loss	(2,419)	(1,925)
Fixed income securities - available for sale	706,193	570,394
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	140,455	66,029
Less than 12 months, Unrealized losses	(2,419)	(1,925)
12 months or more, Unrealized losses		0
Total, Fair value	140,455	66,029
Total, Unrealized losses	(2,419)	(1,925)
Residential mortgage-backed securities [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	1,106,984	958,404
Gross unrealized gain	46,547	40,949
Gross unrealized loss	(3,933)	(4,245)
Fixed income securities - available for sale	1,149,598	995,108
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	163,755	123,782
Less than 12 months, Unrealized losses	(3,273)	(3,081)
12 months or more, Fair value	10,761	22,152
12 months or more, Unrealized losses	(660)	(1,164)
Total, Fair value	174,516	145,934
Total, Unrealized losses	(3,933)	(4,245)
Commercial mortgage-backed securities [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	227,224	136,746
Gross unrealized gain	9,919	8,518
Gross unrealized loss	(1,411)	(36)
Fixed income securities - available for sale	235,732	145,228
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	97,766	0
Less than 12 months, Unrealized losses	(1,411)	0
12 months or more, Fair value		3,084
12 months or more, Unrealized losses		(36)
Total, Fair value	97,766	3,084
Total, Unrealized losses	(1,411)	(36)
Asset-backed securities [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	40,672	12,563
Gross unrealized gain	108	78
Gross unrealized loss	(42)	(75)
Fixed income securities - available for sale	40,738	12,566
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	13,042	9,174
Less than 12 months, Unrealized losses	(42)	(75)
12 months or more, Fair value		0
12 months or more, Unrealized losses		0
Total, Fair value	13,042	9,174
Total, Unrealized losses	(42)	(75)
Foreign government securities [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	252,870	230,348
Gross unrealized gain	8,927	11,537
Gross unrealized loss	(373)	(124)
Fixed income securities - available for sale	261,424	241,761
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	23,023	10,669
Less than 12 months, Unrealized losses	(373)	(124)
12 months or more, Fair value		0
12 months or more, Unrealized losses		0
Total, Fair value	23,023	10,669
Total, Unrealized losses	$ (373)	$ (124)