Investments (Details) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010	Jun. 30, 2011		Jun. 30, 2010	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 6,620,810,000			$ 6,620,810,000			$ 7,369,857,000
Gross unrealized gains	171,358,000			171,358,000			161,988,000
Gross unrealized losses	(45,719,000)	[1]		(45,719,000)	[1]		(73,563,000)	[1]
Fair value	6,746,449,000			6,746,449,000			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	6,618,208,000			6,618,208,000			7,366,808,000
Maturities, Fair Value [Abstract]
Total at end of period	6,746,449,000			6,746,449,000			7,458,282,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	1,528,047,000			1,528,047,000			2,345,030,000
12 months or greater	323,270,000			323,270,000			388,259,000
Total	1,851,317,000			1,851,317,000			2,733,289,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	25,286,000			25,286,000			50,234,000
12 months or greater	20,433,000			20,433,000			23,329,000
Total	45,719,000			45,719,000			73,563,000
Net realized investment gains (losses) and OTTI on investments [Abstract]
Impairment losses	0		0	0		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	873,069,000			873,069,000			1,092,890,000
Gross unrealized gains	18,039,000			18,039,000			16,718,000
Gross unrealized losses	(4,736,000)	[1]		(4,736,000)	[1]		(6,822,000)	[1]
Fair value	886,372,000			886,372,000			1,102,786,000
Maturities, Fair Value [Abstract]
Total at end of period	886,372,000			886,372,000			1,102,786,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	181,707,000			181,707,000			258,235,000
12 months or greater	0			0			0
Total	181,707,000			181,707,000			258,235,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	4,736,000			4,736,000			6,822,000
12 months or greater	0			0			0
Total	4,736,000			4,736,000			6,822,000
Obligations of U.S. States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	3,020,694,000			3,020,694,000			3,549,355,000
Gross unrealized gains	97,550,000			97,550,000			85,085,000
Gross unrealized losses	(33,449,000)	[1]		(33,449,000)	[1]		(54,374,000)	[1]
Fair value	3,084,795,000			3,084,795,000			3,580,066,000
Maturities, Fair Value [Abstract]
Total at end of period	3,084,795,000			3,084,795,000			3,580,066,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	566,735,000			566,735,000			1,160,877,000
12 months or greater	274,603,000			274,603,000			359,629,000
Total	841,338,000			841,338,000			1,520,506,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	14,439,000			14,439,000			32,415,000
12 months or greater	19,010,000			19,010,000			21,959,000
Total	33,449,000			33,449,000			54,374,000
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,348,328,000			2,348,328,000			2,521,275,000
Gross unrealized gains	48,467,000			48,467,000			54,975,000
Gross unrealized losses	(6,530,000)	[1]		(6,530,000)	[1]		(11,291,000)	[1]
Fair value	2,390,265,000			2,390,265,000			2,564,959,000
Maturities, Fair Value [Abstract]
Total at end of period	2,390,265,000			2,390,265,000			2,564,959,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	699,173,000			699,173,000			817,471,000
12 months or greater	43,853,000			43,853,000			28,630,000
Total	743,026,000			743,026,000			846,101,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	5,359,000			5,359,000			9,921,000
12 months or greater	1,171,000			1,171,000			1,370,000
Total	6,530,000			6,530,000			11,291,000
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	164,428,000			164,428,000
Gross unrealized gains	977,000			977,000
Gross unrealized losses	(657,000)	[1]		(657,000)	[1]
Fair value	164,748,000			164,748,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	164,428,000			164,428,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	164,748,000			164,748,000
Total at end of period	164,748,000			164,748,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	69,560,000			69,560,000
12 months or greater	0			0
Total	69,560,000			69,560,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	657,000			657,000
12 months or greater	0			0
Total	657,000			657,000
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	64,282,000			64,282,000			53,845,000
Gross unrealized gains	2,334,000			2,334,000			3,255,000
Gross unrealized losses	0	[1]		0	[1]		0	[1]
Fair value	66,616,000			66,616,000			57,100,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	64,282,000			64,282,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	66,616,000			66,616,000
Total at end of period	66,616,000			66,616,000			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	147,407,000			147,407,000			149,443,000
Gross unrealized gains	3,947,000			3,947,000			1,915,000
Gross unrealized losses	(338,000)	[1]		(338,000)	[1]		(1,031,000)	[1]
Fair value	151,016,000			151,016,000			150,327,000
Maturities, Fair Value [Abstract]
Total at end of period	151,016,000			151,016,000			150,327,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	10,152,000			10,152,000			105,724,000
12 months or greater	4,814,000			4,814,000			0
Total	14,966,000			14,966,000			105,724,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	86,000			86,000			1,031,000
12 months or greater	252,000			252,000			0
Total	338,000			338,000			1,031,000
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	6,618,208,000			6,618,208,000			7,366,808,000
Gross unrealized gains	171,314,000			171,314,000			161,948,000
Gross unrealized losses	(45,710,000)	[1]		(45,710,000)	[1]		(73,518,000)	[1]
Fair value	6,743,812,000			6,743,812,000			7,455,238,000
Maturities, Amortized Cost [Abstract]
Due in one year or less	398,691,000			398,691,000
Due after one year through five years	3,111,422,000			3,111,422,000
Due after five years through ten years	1,234,168,000			1,234,168,000
Due after ten years	1,342,969,000			1,342,969,000
Total debt securities with single maturity date	6,087,250,000			6,087,250,000
Total at end of period	6,618,208,000			6,618,208,000
Maturities, Fair Value [Abstract]
Due in one year or less	400,037,000			400,037,000
Due after one year through five years	3,172,929,000			3,172,929,000
Due after five years through ten years	1,279,285,000			1,279,285,000
Due after ten years	1,373,046,000			1,373,046,000
Total debt securities with single maturity date	6,225,297,000			6,225,297,000
Total at end of period	6,743,812,000			6,743,812,000			7,455,238,000
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,602,000			2,602,000			3,049,000
Gross unrealized gains	44,000			44,000			40,000
Gross unrealized losses	(9,000)	[1]		(9,000)	[1]		(45,000)	[1]
Fair value	2,637,000			2,637,000			3,044,000
Maturities, Fair Value [Abstract]
Total at end of period	2,637,000			2,637,000			3,044,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	720,000			720,000			2,723,000
12 months or greater	0			0			0
Total	720,000			720,000			2,723,000
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	9,000			9,000			45,000
12 months or greater	0			0			0
Total	9,000			9,000			45,000
Auction Rate Securities [Member]
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	302,248,000	[2]		302,248,000	[2]
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	287,151,000	[2]		287,151,000	[2]
Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
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				235,600,000
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			1,021,000			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)			(1,021,000)			(1,021,000)
Ending balance			0			0
Available-for-sale Securities [Member]
Net realized investment gains (losses) and OTTI on investments [Abstract]
Fixed Maturities	21,749,000		31,680,000	27,478,000		58,316,000
Equity securities	7,000		19,000	39,000		57,000
Other	(22,000)		3,000	(22,000)		231,000
Net realized investment gains (losses) and OTTI on investments	21,734,000		31,702,000	27,495,000		58,604,000
Gains on sales	23,553,000		36,608,000	31,945,000		72,588,000
Losses on sales	(1,819,000)		(4,906,000)	(4,450,000)		(7,932,000)
Impairment losses	0		0	0		(6,052,000)
Net realized investment gains (losses) and OTTI on investments	$ 21,734,000		$ 31,702,000	$ 27,495,000		$ 58,604,000

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

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