Investments (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010	Sep. 30, 2011		Sep. 30, 2010	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 6,258,441,000			$ 6,258,441,000			$ 7,369,857,000
Gross unrealized gains	227,037,000			227,037,000			161,988,000
Gross unrealized losses	(27,258,000)			(27,258,000)			(73,563,000)	[1]
Fair value	6,458,220,000			6,458,220,000			7,458,282,000
Percentage of auction rate securities with contractual maturities greater than 10 years (in hundredths)	97.00%			97.00%
Contractual maturity of auction rate securities (in years)	10Y			10Y
Maturities, Amortized Cost [Abstract]
Total at end of period	6,255,817,000			6,255,817,000			7,366,808,000
Maturities, Fair Value [Abstract]
Total at end of period	6,458,220,000			6,458,220,000			7,458,282,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	1,067,944,000			1,067,944,000			2,345,030,000
12 months or greater	284,938,000			284,938,000			388,259,000
Total	1,352,882,000			1,352,882,000			2,733,289,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	11,833,000			11,833,000			50,234,000
12 months or greater	15,425,000			15,425,000			23,329,000
Total	27,258,000			27,258,000			73,563,000
Net realized investment gains (losses) and OTTI on investments [Abstract]
Impairment losses	253,000		0	253,000		6,052,000
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	744,560,000			744,560,000			1,092,890,000
Gross unrealized gains	32,365,000			32,365,000			16,718,000
Gross unrealized losses	(671,000)			(671,000)			(6,822,000)	[1]
Fair value	776,254,000			776,254,000			1,102,786,000
Maturities, Fair Value [Abstract]
Total at end of period	776,254,000			776,254,000			1,102,786,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	72,954,000			72,954,000			258,235,000
12 months or greater	0			0			0
Total	72,954,000			72,954,000			258,235,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	671,000			671,000			6,822,000
12 months or greater	0			0			0
Total	671,000			671,000			6,822,000
Obligations of U.S. States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,888,541,000			2,888,541,000			3,549,355,000
Gross unrealized gains	128,230,000			128,230,000			85,085,000
Gross unrealized losses	(17,139,000)			(17,139,000)			(54,374,000)	[1]
Fair value	2,999,632,000			2,999,632,000			3,580,066,000
Maturities, Fair Value [Abstract]
Total at end of period	2,999,632,000			2,999,632,000			3,580,066,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	222,494,000			222,494,000			1,160,877,000
12 months or greater	260,374,000			260,374,000			359,629,000
Total	482,868,000			482,868,000			1,520,506,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	2,635,000			2,635,000			32,415,000
12 months or greater	14,504,000			14,504,000			21,959,000
Total	17,139,000			17,139,000			54,374,000
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,247,489,000			2,247,489,000			2,521,275,000
Gross unrealized gains	53,439,000			53,439,000			54,975,000
Gross unrealized losses	(7,822,000)			(7,822,000)			(11,291,000)	[1]
Fair value	2,293,106,000			2,293,106,000			2,564,959,000
Maturities, Fair Value [Abstract]
Total at end of period	2,293,106,000			2,293,106,000			2,564,959,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	694,931,000			694,931,000			817,471,000
12 months or greater	24,564,000			24,564,000			28,630,000
Total	719,495,000			719,495,000			846,101,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	6,901,000			6,901,000			9,921,000
12 months or greater	921,000			921,000			1,370,000
Total	7,822,000			7,822,000			11,291,000
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	194,795,000			194,795,000
Gross unrealized gains	1,859,000			1,859,000
Gross unrealized losses	(1,621,000)			(1,621,000)
Fair value	195,033,000			195,033,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	194,795,000			194,795,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	195,033,000			195,033,000
Total at end of period	195,033,000			195,033,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	77,131,000			77,131,000
12 months or greater	0			0
Total	77,131,000			77,131,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	1,621,000			1,621,000
12 months or greater	0			0
Total	1,621,000			1,621,000
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	47,182,000			47,182,000			53,845,000
Gross unrealized gains	2,653,000			2,653,000			3,255,000
Gross unrealized losses	0			0			0	[1]
Fair value	49,835,000			49,835,000			57,100,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	47,182,000			47,182,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	49,835,000			49,835,000
Total at end of period	49,835,000			49,835,000			57,100,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	0			0			0
12 months or greater	0			0			0
Total	0			0			0
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	0			0			0
12 months or greater	0			0			0
Total	0			0			0
Debt Securities Issued by Foreign Sovereign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	133,250,000			133,250,000			149,443,000
Gross unrealized gains	8,415,000			8,415,000			1,915,000
Gross unrealized losses	(4,000)			(4,000)			(1,031,000)	[1]
Fair value	141,661,000			141,661,000			150,327,000
Maturities, Fair Value [Abstract]
Total at end of period	141,661,000			141,661,000			150,327,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	376,000			376,000			105,724,000
12 months or greater	0			0			0
Total	376,000			376,000			105,724,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	4,000			4,000			1,031,000
12 months or greater	0			0			0
Total	4,000			4,000			1,031,000
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	6,255,817,000			6,255,817,000			7,366,808,000
Gross unrealized gains	226,961,000			226,961,000			161,948,000
Gross unrealized losses	(27,257,000)			(27,257,000)			(73,518,000)	[1]
Fair value	6,455,521,000			6,455,521,000			7,455,238,000
Maturities, Amortized Cost [Abstract]
Due in one year or less	1,113,140,000			1,113,140,000
Due after one year through five years	2,659,868,000			2,659,868,000
Due after five years through ten years	905,459,000			905,459,000
Due after ten years	1,035,075,000			1,035,075,000
Total debt securities with single maturity date	5,713,542,000			5,713,542,000
Total at end of period	6,255,817,000			6,255,817,000
Maturities, Fair Value [Abstract]
Due in one year or less	1,118,319,000			1,118,319,000
Due after one year through five years	2,739,392,000			2,739,392,000
Due after five years through ten years	975,609,000			975,609,000
Due after ten years	1,091,339,000			1,091,339,000
Total debt securities with single maturity date	5,924,659,000			5,924,659,000
Total at end of period	6,455,521,000			6,455,521,000			7,455,238,000
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,624,000			2,624,000			3,049,000
Gross unrealized gains	76,000			76,000			40,000
Gross unrealized losses	(1,000)			(1,000)			(45,000)	[1]
Fair value	2,699,000			2,699,000			3,044,000
Maturities, Fair Value [Abstract]
Total at end of period	2,699,000			2,699,000			3,044,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	58,000			58,000			2,723,000
12 months or greater	0			0			0
Total	58,000			58,000			2,723,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	1,000			1,000			45,000
12 months or greater	0			0			0
Total	1,000			1,000			45,000
Auction Rate Securities [Member]
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	300,298,000	[2]		300,298,000	[2]
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	285,994,000	[2]		285,994,000	[2]
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
ARS backed by student loans	286,000,000			286,000,000
Auction process, interest rate reset interval, low (in days)	7			7
Auction process, interest rate reset interval, mid (in days)	28			28
Auction process, interest rate reset interval, high (in days)	35			35
Percentage of ARS held guaranteed by U.S. Department of Education (in hundredths)	97.00%			97.00%
Percentage of ARS portfolio rated AAA/Aaa by one or more of the major rating agencies (in hundredths)	87.00%			87.00%
Number of investments in ARS portfolio	28			28
The par value of ARS sold or called				237,400,000
Percentage of par value received in sale or call of ARS (in hundredths)	99.00%			99.00%
Available-for-sale Securities [Member]
Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance			0			1,021,000
Addition for the amount related to the credit loss for which an OTTI was not previously recognized			0			0
Additional increases to the amount related to the credit loss for which an OTTI was previously recognized			0			0
Reductions for securities sold during the period (realized)			0			(1,021,000)
Ending balance			0			0
Available-for-sale Securities [Member]
Net realized investment gains (losses) and OTTI on investments [Abstract]
Fixed Maturities	10,263,000		24,503,000	37,741,000		82,819,000
Equity securities	12,000		15,000	51,000		72,000
Other	877,000		6,000	855,000		237,000
Net realized investment gains (losses) and OTTI on investments	11,152,000		24,524,000	38,647,000		83,128,000
Gains on sales	12,007,000		26,305,000	43,952,000		98,893,000
Losses on sales	(602,000)		(1,781,000)	(5,052,000)		(9,713,000)
Impairment losses	(253,000)		0	(253,000)		(6,052,000)
Net realized investment gains (losses) and OTTI on investments	$ 11,152,000		$ 24,524,000	$ 38,647,000		$ 83,128,000

[1] At September 30, 2011 and December 31, 2010, there were no other-than-temporary impairment losses recorded in other comprehensive income.

[2] At September 30, 2011, approximately 97% of auction rate securities had a contractual maturity greater than 10 years.