Fair Value Measurements (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Fixed income securities-available for sale
Fixed income securities - available for sale	$ 5,651,189	$ 4,999,440
Other investments	34,088	5,575
Other assets	1,373	857
Total assets measured at fair value	5,686,650	5,005,872
U.S. government and government agency securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	293,344	324,267
U.S. government and government agency securities [Member] \| Level 1 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	185,530	148,217
U.S. government and government agency securities [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	107,814	176,050
U.S. government and government agency securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Fixed income securities of states, municipalities and political subdivisions [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,079,859	1,082,057
Fixed income securities of states, municipalities and political subdivisions [Member] \| Level 1 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Fixed income securities of states, municipalities and political subdivisions [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,079,859	1,082,057
Fixed income securities of states, municipalities and political subdivisions [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Special purpose revenue bonds of states, municipalities and political subdivisions [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,815,959	1,628,059
Special purpose revenue bonds of states, municipalities and political subdivisions [Member] \| Level 1 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Special purpose revenue bonds of states, municipalities and political subdivisions [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,815,959	1,628,059
Special purpose revenue bonds of states, municipalities and political subdivisions [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Corporate fixed income securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	813,606	570,394
Corporate fixed income securities [Member] \| Level 1 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Corporate fixed income securities [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	813,450	570,152
Corporate fixed income securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	156	242
Residential mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,111,321	995,108
Residential mortgage-backed securities [Member] \| Level 1 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Residential mortgage-backed securities [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,111,321	995,108
Residential mortgage-backed securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Commercial mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	249,051	145,228
Commercial mortgage-backed securities [Member] \| Level 1 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Commercial mortgage-backed securities [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	249,051	145,228
Commercial mortgage-backed securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Asset-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	34,617	12,566
Asset-backed securities [Member] \| Level 1 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Asset-backed securities [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	33,555	11,370
Asset-backed securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,062	1,196
Foreign government securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	253,432	241,761
Foreign government securities [Member] \| Level 1 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Foreign government securities [Member] \| Level 2 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	253,432	241,761
Foreign government securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Level 1 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	185,530	148,217
Other investments	34,088	5,575
Other assets	0	0
Total assets measured at fair value	219,618	153,792
Level 2 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	5,464,441	4,849,785
Other investments	0	0
Other assets	0	0
Total assets measured at fair value	5,464,441	4,849,785
Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,218	1,438
Other investments	0	0
Other assets	1,373	857
Total assets measured at fair value	$ 2,591	$ 2,295

For an unclassified balance sheet, total of debt securities categorized neither as held-to-maturity nor trading.

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+ Details

- Definition

For an unclassified balance sheet, this item represents equity securities categorized neither as held-to-maturity nor trading.

+ References

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+ Details