Investments (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011	Sep. 30, 2012		Sep. 30, 2011	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,796,434,000			$ 4,796,434,000			$ 5,703,560,000
Gross Unrealized Gains	136,272,000			136,272,000			134,673,000
Gross Unrealized Losses	(5,942,000)	[1]		(5,942,000)	[1]		(14,586,000)	[1]
Fair Value	4,926,764,000			4,926,764,000			5,823,647,000
Maturities, Amortized Cost [Abstract]
Due in one year or less	780,615,000			780,615,000
Due after one year through five years	1,902,433,000			1,902,433,000
Due after five years through ten years	869,795,000			869,795,000
Due after ten years	421,798,000			421,798,000
Total debt securities with single maturity date	3,974,641,000			3,974,641,000
Total at end of period	4,793,698,000			4,793,698,000			5,700,894,000
Maturities, Fair Value [Abstract]
Due in one year or less	783,234,000			783,234,000
Due after one year through five years	1,952,796,000			1,952,796,000
Due after five years through ten years	920,848,000			920,848,000
Due after ten years	432,197,000			432,197,000
Total debt securities with single maturity date	4,089,075,000			4,089,075,000
Total at end of period	4,923,846,000			4,923,846,000
Contractual maturity of auction rate securities	10 years			10 years
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	284,298,000			284,298,000			1,077,710,000
12 months or greater	98,284,000			98,284,000			166,172,000
Total investment portfolio	382,582,000			382,582,000			1,243,882,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	1,514,000			1,514,000			7,873,000
12 months or greater	4,428,000			4,428,000			6,713,000
Total investment portfolio	5,942,000			5,942,000			14,586,000
Fair value of ARS backed by student loans	111,000,000			111,000,000			170,000,000
Auction process, interest rate reset interval, low				7 days
Auction process, interest rate reset interval, mid				28 days
Auction process, interest rate reset interval, high				35 days
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)	66.00%			66.00%
Number of investments in ARS portfolio	13			13
The par value of ARS sold or called				422,000,000
Percentage of par value received in sale or call of ARS (in hundredths)	96.00%			96.00%
Other than Temporary Impairments				300,000
Gain (Loss) on Investments [Line Items]
Impairment losses	0		(253,000)	(339,000)		(253,000)
Available-for-sale Securities [Member]
Gain (Loss) on Investments [Line Items]
Net realized investment gains (losses) and OTTI on investments	6,184,000		11,152,000	110,017,000		38,647,000
Gains on sales	10,559,000		12,007,000	118,599,000		43,952,000
Losses on sales	(4,375,000)		(602,000)	(8,243,000)		(5,052,000)
Impairment losses	0		(253,000)	(339,000)		(253,000)
Net realized investment gains (losses) and OTTI on investments	6,184,000		11,152,000	110,017,000		38,647,000
Available-for-sale Securities [Member] | Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Net realized investment gains (losses) and OTTI on investments	8,901,000		10,263,000	110,335,000		37,741,000
Net realized investment gains (losses) and OTTI on investments	8,901,000		10,263,000	110,335,000		37,741,000
Available-for-sale Securities [Member] | Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Net realized investment gains (losses) and OTTI on investments	30,000		12,000	424,000		51,000
Net realized investment gains (losses) and OTTI on investments	30,000		12,000	424,000		51,000
Available-for-sale Securities [Member] | Other [Member]
Gain (Loss) on Investments [Line Items]
Net realized investment gains (losses) and OTTI on investments	(2,747,000)		877,000	(742,000)		855,000
Net realized investment gains (losses) and OTTI on investments	(2,747,000)		877,000	(742,000)		855,000
U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	214,845,000			214,845,000			592,108,000
Gross Unrealized Gains	4,438,000			4,438,000			4,965,000
Gross Unrealized Losses	(20,000)	[1]		(20,000)	[1]		(36,000)	[1]
Fair Value	219,263,000			219,263,000			597,037,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	8,953,000			8,953,000			78,546,000
12 months or greater	0			0			0
Total investment portfolio	8,953,000			8,953,000			78,546,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	20,000			20,000			36,000
12 months or greater	0			0			0
Total investment portfolio	20,000			20,000			36,000
Obligations of U.S. states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,228,688,000			1,228,688,000			2,255,192,000
Gross Unrealized Gains	47,441,000			47,441,000			74,918,000
Gross Unrealized Losses	(4,097,000)	[1]		(4,097,000)	[1]		(6,639,000)	[1]
Fair Value	1,272,032,000			1,272,032,000			2,323,471,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	66,542,000			66,542,000			188,879,000
12 months or greater	64,227,000			64,227,000			137,965,000
Total investment portfolio	130,769,000			130,769,000			326,844,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	856,000			856,000			837,000
12 months or greater	3,241,000			3,241,000			5,802,000
Total investment portfolio	4,097,000			4,097,000			6,639,000
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,512,767,000			2,512,767,000			2,007,720,000
Gross Unrealized Gains	52,240,000			52,240,000			32,750,000
Gross Unrealized Losses	(1,569,000)	[1]		(1,569,000)	[1]		(7,619,000)	[1]
Fair Value	2,563,438,000			2,563,438,000			2,032,851,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	163,008,000			163,008,000			689,396,000
12 months or greater	34,057,000			34,057,000			28,174,000
Total investment portfolio	197,065,000			197,065,000			717,570,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	382,000			382,000			6,709,000
12 months or greater	1,187,000			1,187,000			910,000
Total investment portfolio	1,569,000			1,569,000			7,619,000
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	446,979,000			446,979,000			441,589,000
Gross Unrealized Gains	10,160,000			10,160,000			4,113,000
Gross Unrealized Losses	(229,000)	[1]		(229,000)	[1]		(285,000)	[1]
Fair Value	456,910,000			456,910,000			445,417,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	446,979,000			446,979,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	456,910,000			456,910,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	37,600,000			37,600,000			120,405,000
12 months or greater	0			0			0
Total investment portfolio	37,600,000			37,600,000			120,405,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	229,000			229,000			285,000
12 months or greater	0			0			0
Total investment portfolio	229,000			229,000			285,000
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	256,794,000			256,794,000			257,530,000
Gross Unrealized Gains	9,996,000			9,996,000			7,404,000
Gross Unrealized Losses	0	[1]		0	[1]		0	[1]
Fair Value	266,790,000			266,790,000			264,934,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	256,794,000			256,794,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	266,790,000			266,790,000
Debt securities issued by foreign sovereign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	133,625,000			133,625,000			146,755,000
Gross Unrealized Gains	11,815,000			11,815,000			10,441,000
Gross Unrealized Losses	(27,000)	[1]		(27,000)	[1]		(6,000)	[1]
Fair Value	145,413,000			145,413,000			157,190,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	8,195,000			8,195,000			484,000
12 months or greater	0			0			0
Total investment portfolio	8,195,000			8,195,000			484,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	27,000			27,000			6,000
12 months or greater	0			0			0
Total investment portfolio	27,000			27,000			6,000
Total debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,793,698,000			4,793,698,000			5,700,894,000
Gross Unrealized Gains	136,090,000			136,090,000			134,591,000
Gross Unrealized Losses	(5,942,000)	[1]		(5,942,000)	[1]		(14,585,000)	[1]
Fair Value	4,923,846,000			4,923,846,000			5,820,900,000
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,736,000			2,736,000			2,666,000
Gross Unrealized Gains	182,000			182,000			82,000
Gross Unrealized Losses	0	[1]		0	[1]		(1,000)	[1]
Fair Value	2,918,000			2,918,000			2,747,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months							0
12 months or greater							33,000
Total investment portfolio							33,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months							0
12 months or greater							1,000
Total investment portfolio							1,000
Auction Rate Securities [Member]
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	115,284,000	[2]		115,284,000	[2]
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	$ 111,071,000	[2]		$ 111,071,000	[2]

[1] At September 30, 2012 and December 31, 2011, there were no other-than-temporary impairment losses recorded in other comprehensive income.

[2] At September 30, 2012, all of the auction rate securities had a contractual maturity greater than 10 years.