Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fixed income securities-available for sale
Fixed income securities - available for sale	$ 5,718,834	$ 4,999,440
Other investments	35,720	5,575
Other assets	1,516	857
Total assets measured at fair value	5,756,070	5,005,872
Level 1 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	201,582	148,217
Other investments	35,720	5,575
Other assets	0	0
Total assets measured at fair value	237,302	153,792
Level 2 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	5,516,082	4,849,785
Other investments	0	0
Other assets	0	0
Total assets measured at fair value	5,516,082	4,849,785
Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,170	1,438
Other investments	0	0
Other assets	1,516	857
Total assets measured at fair value	2,686	2,295
U.S. government and government agency securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	295,679	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	201,582	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	94,097	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	1,085,341	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	1,085,341	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	1,863,888	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	1,863,888	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	846,333	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	846,178	570,152
Corporate fixed income securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	155	242
Residential mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,100,086	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	1,100,086	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	256,124	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	256,124	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	34,746	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	33,731	11,370
Asset-backed securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,015	1,196
Foreign government securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	236,637	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	236,637
Foreign government securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	$ 0	$ 0

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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+ Details

- Definition

Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

+ References+ Details