INVESTMENTS (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2005 investments
Available-for-sale securities [Abstract]
Amortized cost	$ 2,644,753,000		$ 2,633,503,000
Unrealized gain	111,913,000		106,908,000
Unrealized loss	(947,000)		(10,212,000)
Fair value	2,755,719,000		2,730,199,000
Fair value of equity method Investments	24,605,000	[1]	21,315,000	[1]
Fair value of investments	2,780,324,000		2,751,514,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	204,949,000		382,284,000
12 months or more	1,738,000		43,000
Total	206,687,000		382,327,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(944,000)		(10,211,000)
12 months or more	(3,000)		(1,000)
Total	(947,000)		(10,212,000)
Available-for-sale securities, debt maturities, amortized cost basis [Abstract]
Within 1 year	362,116,000		315,362,000
1 to 5 years	798,143,000		984,503,000
5 to 10 years	652,941,000		536,577,000
Over 10 years	831,553,000		797,061,000
Amortized Cost	2,644,753,000		2,633,503,000
Available-for-sale securities, debt maturities, fair value [Abstract]
Within 1 year	363,559,000		317,067,000
1 to 5 years	822,448,000		1,006,221,000
5 to 10 years	693,504,000		574,207,000
Over 10 years	876,208,000		832,704,000
Fair value	2,755,719,000		2,730,199,000
Number of investments acquired						8
Equity method investments	23,700,000
Interest in limited partnership, minimum (in hundredths)	20.00%
Interest in limited partnership, maximum (in hundredths)	25.00%
Ownership percentage of partnership (in hundredths)	10.00%
Gross investment gains on sale of investments	41,500,000		17,400,000		15,500,000
Gross investment losses on sale of investments	1,100,000		400,000		4,500,000
State and municipal bonds [Member]
Available-for-sale securities [Abstract]
Amortized cost	1,176,016,000		970,746,000
Unrealized gain	78,272,000		62,215,000
Unrealized loss	(499,000)		(7,000)
Fair value	1,253,789,000		1,032,954,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	61,342,000		9,436,000
12 months or more	0		0
Total	61,342,000		9,436,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(499,000)		(7,000)
12 months or more	0		0
Total	(499,000)		(7,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Fair value	1,253,789,000		1,032,954,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	78,264,000		88,934,000
Unrealized gain	669,000		2,410,000
Unrealized loss	(2,000)		(4,000)
Fair value	78,931,000		91,340,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	2,458,000		4,932,000
12 months or more	1,065,000		0
Total	3,523,000		4,932,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(1,000)		(4,000)
12 months or more	(1,000)		0
Total	(2,000)		(4,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Fair value	78,931,000		91,340,000
Government-sponsored enterprise securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	72,394,000	[2]	140,595,000	[2]
Unrealized gain	1,139,000	[2]	2,694,000	[2]
Unrealized loss	(1,000)	[2]	(11,000)	[2]
Fair value	73,532,000	[2]	143,278,000	[2]
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	15,714,000		12,495,000
12 months or more	0		0
Total	15,714,000		12,495,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(1,000)		(11,000)
12 months or more	0		0
Total	(1,000)		(11,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Fair value	73,532,000	[2]	143,278,000	[2]
Residential mortgage-backed securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	302,012,000	[3]	354,713,000	[3]
Unrealized gain	10,703,000	[3]	14,097,000	[3]
Unrealized loss	(74,000)	[3]	(12,000)	[3]
Fair value	312,641,000	[3]	368,798,000	[3]
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	23,861,000		5,127,000
12 months or more	59,000		43,000
Total	23,920,000		5,170,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(73,000)		(11,000)
12 months or more	(1,000)		(1,000)
Total	(74,000)		(12,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Fair value	312,641,000	[3]	368,798,000	[3]
Commercial mortgage-backed securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	21,416,000		13,801,000
Unrealized gain	193,000		1,024,000
Unrealized loss	(19,000)		0
Fair value	21,590,000		14,825,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	7,701,000		0
12 months or more	0		0
Total	7,701,000		0
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(19,000)		0
12 months or more	0		0
Total	(19,000)		0
Available-for-sale securities, debt maturities, fair value [Abstract]
Fair value	21,590,000		14,825,000
Asset-backed securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	23,421,000	[4]	12,840,000	[4]
Unrealized gain	211,000	[4]	664,000	[4]
Unrealized loss	(6,000)	[4]	0	[4]
Fair value	23,626,000	[4]	13,504,000	[4]
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	14,492,000		0
12 months or more	0		0
Total	14,492,000		0
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(6,000)		0
12 months or more	0		0
Total	(6,000)		0
Available-for-sale securities, debt maturities, fair value [Abstract]
Fair value	23,626,000	[4]	13,504,000	[4]
Corporate debt and other securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	971,230,000		1,051,874,000
Unrealized gain	20,726,000		23,804,000
Unrealized loss	(346,000)		(10,178,000)
Fair value	991,610,000		1,065,500,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	79,381,000		350,294,000
12 months or more	614,000		0
Total	79,995,000		350,294,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(345,000)		(10,178,000)
12 months or more	(1,000)		0
Total	(346,000)		(10,178,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Fair value	$ 991,610,000		$ 1,065,500,000

[1] Includes investments in entities accounted for under the equity method of accounting and therefore are presented at their carrying value.

[2] Includes FDIC-insured Temporary Liquidity Guarantee Program (“TLGP”) securities. As of December 31, 2012, the Company no longer held any TLGP securities.

[3] Includes Agency pass-through securities, with the timely payment of principal and interest guaranteed.

[4] Includes auto loans, credit card debt, and rate reduction bonds.