Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fixed income securities-available for sale
Fixed maturity securities - available for sale	$ 6,315,365	$ 5,718,834
Equity securities-available for sale, at fair value (cost: $197,469)	202,864	0
Held-to-maturity Securities, Fair Value	0	163,136
Short-term investments*	160,138	133,917
Other Investments	34,044	35,897
Other assets	206	1,516
Total assets measured at fair value	6,712,617	6,053,123
Notes payable	595,804	505,671
Accounts payable and accrued liabilities - forward contract	1,705
Total liabilities measured at fair value	597,509
Fair Value, Inputs, Level 1 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	193,051	201,582
Equity securities-available for sale, at fair value (cost: $197,469)	202,864
Short-term investments*	81,947	67,288
Other Investments	34,044	35,720
Total assets measured at fair value	511,906	304,590
Fair Value, Inputs, Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	6,122,152	5,516,082
Held-to-maturity Securities, Fair Value		163,136
Short-term investments*	78,191	66,629
Total assets measured at fair value	6,200,343	5,745,847
Notes payable	595,804	505,671
Accounts payable and accrued liabilities - forward contract	1,705
Total liabilities measured at fair value	597,509
Fair Value, Inputs, Level 3 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	162	1,170
Other assets	206	1,516
Total assets measured at fair value	368	2,686
US Government Agencies Debt Securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	232,036	295,679
US Government Agencies Debt Securities [Member] \| Fair Value, Inputs, Level 1 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	193,051	201,582
US Government Agencies Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	38,985	94,097
US States and Political Subdivisions Debt Securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	1,079,175	1,085,341
US States and Political Subdivisions Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	1,079,175	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	2,076,017	1,863,888
Special Purpose Revenue Bonds Of States Municipalities And Political Subdivisions [Member] \| Fair Value, Inputs, Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	2,076,017	1,863,888
Corporate Debt Securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	1,217,307	846,333
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	1,217,145	846,178
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 3 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	162	155
Residential Mortgage Backed Securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	841,542	1,100,086
Residential Mortgage Backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	841,542	1,100,086
Commercial Mortgage Backed Securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	524,100	256,124
Commercial Mortgage Backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	524,100	256,124
Asset-backed Securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	47,724	34,746
Asset-backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	47,724	33,731
Asset-backed Securities [Member] \| Fair Value, Inputs, Level 3 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale		1,015
Foreign Government Debt Securities [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	297,464	236,637
Foreign Government Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fixed income securities-available for sale
Fixed maturity securities - available for sale	$ 297,464	$ 236,637

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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