Fair Value Measurements (Details) (USD $)	9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2012		Dec. 31, 2011		Sep. 30, 2012 Obligations of U.S. States and Political Subdivisions [Member]	Sep. 30, 2011 Obligations of U.S. States and Political Subdivisions [Member]	Sep. 30, 2012 Obligations of U.S. States and Political Subdivisions [Member]	Sep. 30, 2011 Obligations of U.S. States and Political Subdivisions [Member]	Sep. 30, 2012 Corporate Debt Securities [Member]	Sep. 30, 2011 Corporate Debt Securities [Member]	Sep. 30, 2012 Corporate Debt Securities [Member]	Sep. 30, 2011 Corporate Debt Securities [Member]	Sep. 30, 2012 Equity Securities [Member]	Sep. 30, 2011 Equity Securities [Member]	Sep. 30, 2012 Equity Securities [Member]	Sep. 30, 2011 Equity Securities [Member]	Sep. 30, 2012 Total Investments [Member]	Sep. 30, 2011 Total Investments [Member]	Sep. 30, 2012 Total Investments [Member]	Sep. 30, 2011 Total Investments [Member]	Sep. 30, 2012 Real Estate Acquired [Member]	Sep. 30, 2011 Real Estate Acquired [Member]	Sep. 30, 2012 Real Estate Acquired [Member]	Sep. 30, 2011 Real Estate Acquired [Member]	Sep. 30, 2012 Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]		Dec. 31, 2011 Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]		Sep. 30, 2012 Significant Other Observable Inputs (Level 2) [Member]		Dec. 31, 2011 Significant Other Observable Inputs (Level 2) [Member]		Sep. 30, 2012 Significant Unobservable Inputs (Level 3) [Member]		Dec. 31, 2011 Significant Unobservable Inputs (Level 3) [Member]		Sep. 30, 2012 U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member]	Dec. 31, 2011 U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member]	Sep. 30, 2012 U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2011 U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2012 U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2011 U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2012 U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2011 U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2012 Obligations of U.S. States and Political Subdivisions [Member]	Dec. 31, 2011 Obligations of U.S. States and Political Subdivisions [Member]	Sep. 30, 2012 Obligations of U.S. States and Political Subdivisions [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2011 Obligations of U.S. States and Political Subdivisions [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2012 Obligations of U.S. States and Political Subdivisions [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2011 Obligations of U.S. States and Political Subdivisions [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2012 Obligations of U.S. States and Political Subdivisions [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2011 Obligations of U.S. States and Political Subdivisions [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2012 Corporate Debt Securities [Member]	Dec. 31, 2011 Corporate Debt Securities [Member]	Sep. 30, 2012 Corporate Debt Securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2011 Corporate Debt Securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2012 Corporate Debt Securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2011 Corporate Debt Securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2012 Corporate Debt Securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2011 Corporate Debt Securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2012 Residential mortgage-backed securities [Member]	Dec. 31, 2011 Residential mortgage-backed securities [Member]	Sep. 30, 2012 Residential mortgage-backed securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2011 Residential mortgage-backed securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2012 Residential mortgage-backed securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2011 Residential mortgage-backed securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2012 Residential mortgage-backed securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2011 Residential mortgage-backed securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2012 Commercial mortgage-backed securities [Member]	Dec. 31, 2011 Commercial mortgage-backed securities [Member]	Sep. 30, 2012 Commercial mortgage-backed securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2011 Commercial mortgage-backed securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2012 Commercial mortgage-backed securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2011 Commercial mortgage-backed securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2012 Commercial mortgage-backed securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2011 Commercial mortgage-backed securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2012 Debt securities issued by foreign sovereign governments [Member]	Dec. 31, 2011 Debt securities issued by foreign sovereign governments [Member]	Sep. 30, 2012 Debt securities issued by foreign sovereign governments [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2011 Debt securities issued by foreign sovereign governments [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2012 Debt securities issued by foreign sovereign governments [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2011 Debt securities issued by foreign sovereign governments [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2012 Debt securities issued by foreign sovereign governments [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2011 Debt securities issued by foreign sovereign governments [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2012 Total debt securities [Member]	Dec. 31, 2011 Total debt securities [Member]	Sep. 30, 2012 Total debt securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2011 Total debt securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2012 Total debt securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2011 Total debt securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2012 Total debt securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2011 Total debt securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2012 Equity securities [Member]	Dec. 31, 2011 Equity securities [Member]	Sep. 30, 2012 Equity securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2011 Equity securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2012 Equity securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2011 Equity securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2012 Equity securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2011 Equity securities [Member] Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements [Abstract]
Discount rates used for the DCF model, minimum (in hundredths)	2.21%
Discount rate used for the DCF model, maximum (in hundredths)	3.71%
Percentage of change in discount rate (in hundredths)	1.00%
Change in value of ARS with change in discount rate assumption	$ 2,400,000
Change in years to liquidity assumption	2 years
Change in value of ARS with change in expected term	4,600,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	4,926,764,000		5,823,647,000																						367,273,000		748,894,000		4,444,450,000		4,899,978,000		115,041,000		174,775,000		219,263,000	597,037,000	219,263,000	597,037,000	0	0	0	0	1,272,032,000	2,323,471,000	0	0	1,198,237,000	2,209,245,000	73,795,000	114,226,000	2,563,438,000	2,032,851,000	0	1,455,000	2,522,513,000	1,971,168,000	40,925,000	60,228,000	456,910,000	445,417,000	0	0	456,910,000	445,417,000	0	0	266,790,000	264,934,000	0	0	266,790,000	264,934,000	0	0	145,413,000	157,190,000	145,413,000	147,976,000	0	9,214,000	0	0	4,923,846,000	5,820,900,000	364,676,000	746,468,000	4,444,450,000	4,899,978,000	114,720,000	174,454,000	2,918,000	2,747,000	2,597,000	2,426,000	0	0	321,000	321,000
Real estate acquired	3,097,000	[1]	1,621,000	[1]																					0	[1]	0	[1]	0	[1]	0	[1]	3,097,000	[1]	1,621,000	[1]
Reconciliation of assets measured at fair value using significant unobservable inputs (Level 3) [Roll Forward]
Balance at beginning of period					83,981,000	223,402,000	114,226,000	295,690,000	40,857,000	70,039,000	60,228,000	70,053,000	321,000	321,000	321,000	321,000	125,159,000	293,762,000	174,775,000	366,064,000	3,074,000	2,828,000	1,621,000	6,220,000
Total realized/unrealized gains (losses) [Abstract]
Included in earnings and reported as realized investment gains (losses), net					(467,000)		(2,992,000)		0		(1,081,000)		0		0		(467,000)		(4,073,000)		0		0
Included in earnings and reported as impairment losses, net	300,000				0	0	0	0	0	(200,000)	(339,000)	(200,000)	0	0	0	0	0	(200,000)	(339,000)	(200,000)	0	0	0	0
Included in earnings and reported as losses incurred, net					0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	(309,000)	(85,000)	(774,000)	(180,000)
Included in other comprehensive income					971,000	342,000	1,727,000	(845,000)	68,000	451,000	423,000	437,000	0	0	0	0	1,039,000	793,000	2,150,000	(408,000)	0	0	0	0
Purchases					0	0	27,000	0	0	0	0	0	0	0	0	0	0	0	27,000	0	2,718,000	1,148,000	8,688,000	3,944,000
Sales					(10,690,000)	(2,537,000)	(39,193,000)	(73,638,000)	0	0	(18,306,000)	0	0	0	0	0	(10,690,000)	(2,537,000)	(57,499,000)	(73,638,000)	(2,386,000)	(1,567,000)	(6,438,000)	(7,660,000)
Transfers into Level 3					0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Transfers out of Level 3					0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Balance at end of period					73,795,000	221,207,000	73,795,000	221,207,000	40,925,000	70,290,000	40,925,000	70,290,000	321,000	321,000	321,000	321,000	115,041,000	291,818,000	115,041,000	291,818,000	3,097,000	2,324,000	3,097,000	2,324,000
Amount of total losses included in earnings for the period attributable to the change in unrealized losses on assets still held at period end					$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0

[1] Real estate acquired through claim settlement, which is held for sale, is reported in Other Assets on the consolidated balance sheet.