Fair Value Measurements (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Fixed income securities-available for sale
Fixed income securities - available for sale	$ 5,434,761	$ 4,999,440
Other investments	31,742	5,575
Other assets	1,242	857
Total assets measured at fair value	5,467,745	5,005,872
U.S. government and government agency securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	363,355	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	209,016	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	154,339	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	1,047,341	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	1,047,341	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	1,630,380	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	1,630,380	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	706,193	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	706,037	570,152
Corporate fixed income securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	156	242
Asset-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	40,738	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	39,650	11,370
Asset-backed securities [Member] \| Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,088	1,196
Foreign government securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	261,424	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	261,424	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	209,016	148,217
Other investments	31,742	5,575
Other assets	0	0
Total assets measured at fair value	240,758	153,792
Level 1 [Member] \| Residential mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Level 1 [Member] \| Commercial mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Level 2 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	5,224,501	4,849,785
Other investments	0	0
Other assets	0	0
Total assets measured at fair value	5,224,501	4,849,785
Level 2 [Member] \| Residential mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,149,598	995,108
Level 2 [Member] \| Commercial mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	235,732	145,228
Level 3 [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,244	1,438
Other investments	0	0
Other assets	1,242	857
Total assets measured at fair value	2,486	2,295
Level 3 [Member] \| Residential mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Level 3 [Member] \| Commercial mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	0	0
Residential mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	1,149,598	995,108
Commercial mortgage-backed securities [Member]
Fixed income securities-available for sale
Fixed income securities - available for sale	$ 235,732	$ 145,228

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Publisher FASB
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-Publisher FASB
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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
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 3
-Subparagraph c
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 19
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ 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