Investments (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 Investment		Dec. 31, 2011		Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,188,734,000		$ 5,703,560,000
Gross Unrealized Gains	46,793,000		134,673,000
Gross Unrealized Losses	(5,252,000)	[1]	(14,586,000)	[1]
Fair Value	4,230,275,000		5,823,647,000
Percentage of foreign investments held in government and semi-government securities (in hundredths)	88.00%
Percentage of foreign investments held in corporate securities (in hundredths)	11.00%
Percentage Of Foreign Investments Held In Cash equivalents (in hundredths)	1.00%
Maturities, Amortized Cost [Abstract]
Due in one year or less	1,097,193,000
Due after one year through five years	1,673,962,000
Due after five years through ten years	480,319,000
Due after ten years	315,765,000
Total debt securities with single maturity date	3,567,239,000
Total at end of period	4,185,937,000		5,700,894,000
Maturities, Fair Value [Abstract]
Due in one year or less	1,099,383,000
Due after one year through five years	1,696,376,000
Due after five years through ten years	491,933,000
Due after ten years	321,282,000
Total debt securities with single maturity date	3,608,974,000
Total at end of period	4,227,339,000
Percentage of auction rate securities with contractual maturities greater than 10 years (in hundredths)	100.00%
Contractual maturity of auction rate securities	10 years
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	893,029,000		1,077,710,000
12 months or greater	24,298,000		166,172,000
Total investment portfolio	917,327,000		1,243,882,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	4,917,000		7,873,000
12 months or greater	335,000		6,713,000
Total investment portfolio	5,252,000		14,586,000
Number of investments in auction rate portfolio	2
Recognized other than temporary impairments	2,310,000		715,000		9,644,000
Investments on deposit with various states	21,400,000		22,300,000
U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	863,282,000		592,108,000
Gross Unrealized Gains	3,040,000		4,965,000
Gross Unrealized Losses	(71,000)	[1]	(36,000)	[1]
Fair Value	866,251,000		597,037,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	24,094,000		78,546,000
12 months or greater	0		0
Total investment portfolio	24,094,000		78,546,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	71,000		36,000
12 months or greater	0		0
Total investment portfolio	71,000		36,000
Obligations of U.S. States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	795,935,000		2,255,192,000
Gross Unrealized Gains	16,965,000		74,918,000
Gross Unrealized Losses	(506,000)	[1]	(6,639,000)	[1]
Fair Value	812,394,000		2,323,471,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	156,111,000		188,879,000
12 months or greater	1,006,000		137,965,000
Total investment portfolio	157,117,000		326,844,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	505,000		837,000
12 months or greater	1,000		5,802,000
Total investment portfolio	506,000		6,639,000
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,792,646,000	[2]	2,007,720,000	[2]
Gross Unrealized Gains	15,470,000	[2]	32,750,000	[2]
Gross Unrealized Losses	(2,739,000)	[1],[2]	(7,619,000)	[1],[2]
Fair Value	1,805,377,000	[2]	2,032,851,000	[2]
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	310,440,000		689,396,000
12 months or greater	3,353,000		28,174,000
Total investment portfolio	313,793,000		717,570,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	2,737,000		6,709,000
12 months or greater	2,000		910,000
Total investment portfolio	2,739,000		7,619,000
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	451,352,000		441,589,000
Gross Unrealized Gains	871,000		4,113,000
Gross Unrealized Losses	(1,314,000)	[1]	(285,000)	[1]
Fair Value	450,909,000		445,417,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	451,352,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	450,909,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	315,000,000		120,405,000
12 months or greater	19,939,000		0
Total investment portfolio	334,939,000		120,405,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	982,000		285,000
12 months or greater	332,000		0
Total investment portfolio	1,314,000		285,000
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	150,232,000		257,530,000
Gross Unrealized Gains	524,000		7,404,000
Gross Unrealized Losses	(414,000)	[1]	0	[1]
Fair Value	150,342,000		264,934,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	150,232,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	150,342,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	72,689,000
12 months or greater	0
Total investment portfolio	72,689,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	414,000
12 months or greater	0
Total investment portfolio	414,000
Debt securities issued by foreign sovereign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	132,490,000		146,755,000
Gross Unrealized Gains	9,784,000		10,441,000
Gross Unrealized Losses	(208,000)	[1]	(6,000)	[1]
Fair Value	142,066,000		157,190,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	14,695,000		484,000
12 months or greater	0		0
Total investment portfolio	14,695,000		484,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	208,000		6,000
12 months or greater	0		0
Total investment portfolio	208,000		6,000
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,185,937,000		5,700,894,000
Gross Unrealized Gains	46,654,000		134,591,000
Gross Unrealized Losses	(5,252,000)	[1]	(14,585,000)	[1]
Fair Value	4,227,339,000		5,820,900,000
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,797,000		2,666,000
Gross Unrealized Gains	139,000		82,000
Gross Unrealized Losses	0	[1]	(1,000)	[1]
Fair Value	2,936,000		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	17,114,000	[3]
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	17,114,000	[3]
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Fair value of ARS backed by student loans	17,000,000		170,000,000
Percentage of ARS held guaranteed by U.S. Department of Education (in hundredths)	97.00%
Percentage of ARS portfolio rated AAA/Aaa by one or more of the major rating agencies (in hundredths)	100.00%
Corporate Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 324,000,000		$ 199,000,000

[1] There were no other-than-temporary impairment losses recorded in other comprehensive income at December 31, 2012 and 2011.

[2] Includes investments in corporate asset-backed securities with a fair value of $324 million and $199 million at December 31, 2012 and 2011, respectively.

[3] At December 31, 2012, 100% of auction rate securities had a contractual maturity greater than 10 years.