Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fixed income securities-available for sale
Fixed maturity securities - available for sale	$ 6,097,866	$ 5,718,834
Equity securities-available for sale, at fair value (cost: $92,580)	93,660	0
Held-to-maturity Securities, Fair Value	0	163,136
Short-term Investments	217,087	133,917
Other Investments	37,720	35,897
Other assets	1,847	1,516
Total assets measured at fair value	6,448,180	6,053,123
Notes payable	635,479	505,671
Accounts payable and accrued liabilities - forward contract	926
Total liabilities measured at fair value	636,405
Level 1 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	188,757	201,582
Equity securities-available for sale, at fair value (cost: $92,580)	93,660
Short-term Investments	129,828	67,288
Other Investments	37,720	35,720
Total assets measured at fair value	449,965	304,590
Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	5,908,950	5,516,082
Held-to-maturity Securities, Fair Value		163,136
Short-term Investments	87,259	66,629
Total assets measured at fair value	5,996,209	5,745,847
Notes payable	635,479	505,671
Accounts payable and accrued liabilities - forward contract	926
Total liabilities measured at fair value	636,405
Level 3 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	159	1,170
Other assets	1,847	1,516
Total assets measured at fair value	2,006	2,686
U.S. government and government agency securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	228,971	295,679
U.S. government and government agency securities [Member] \| Level 1 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	188,757	201,582
U.S. government and government agency securities [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	40,214	94,097
Fixed income securities of states, municipalities and political subdivisions [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	1,073,868	1,085,341
Fixed income securities of states, municipalities and political subdivisions [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	1,073,868	1,085,341
Special purpose revenue bonds of states, municipalities and political subdivisions [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	2,007,548	1,863,888
Special purpose revenue bonds of states, municipalities and political subdivisions [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	2,007,548	1,863,888
Corporate fixed income securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	1,129,657	846,333
Corporate fixed income securities [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	1,129,498	846,178
Corporate fixed income securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale		155
Total assets measured at fair value	159
Residential mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	902,832	1,100,086
Residential mortgage-backed securities [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	902,832	1,100,086
Commercial mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	416,700	256,124
Commercial mortgage-backed securities [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	416,700	256,124
Asset-backed securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	46,182	34,746
Asset-backed securities [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	46,182	33,731
Asset-backed securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale		1,015
Foreign government securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	292,108	236,637
Foreign government securities [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	$ 292,108	$ 236,637

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