INVESTMENTS (Details) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011	Jun. 30, 2012		Jun. 30, 2011	Dec. 31, 2011
Available-for-sale securities [Abstract]
Amortized cost	$ 2,585,502,000			$ 2,585,502,000			$ 2,633,503,000
Unrealized gain	110,409,000			110,409,000			106,908,000
Unrealized loss	(2,582,000)			(2,582,000)			(10,212,000)
Fair value	2,693,329,000			2,693,329,000			2,730,199,000
Fair value of equity method investments	19,582,000	[1]		19,582,000	[1]		21,315,000	[1]
Fair value of investments	2,712,911,000			2,712,911,000			2,751,514,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	163,807,000			163,807,000			382,284,000
12 months or more	101,069,000			101,069,000			43,000
Total	264,876,000			264,876,000			382,327,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(1,238,000)			(1,238,000)			(10,211,000)
12 months or more	(1,344,000)			(1,344,000)			(1,000)
Total	(2,582,000)			(2,582,000)			(10,212,000)
Available-for-sale securities, debt maturities, amortized cost basis [Abstract]
Within 1 year	354,206,000			354,206,000			315,362,000
1 to 5 years	761,127,000			761,127,000			984,503,000
5 to 10 years	589,954,000			589,954,000			536,577,000
Over 10 years	880,215,000			880,215,000			797,061,000
Total	2,585,502,000			2,585,502,000			2,633,503,000
Available-for-sale securities, debt maturities, fair value [Abstract]
Within 1 year	355,756,000			355,756,000			317,067,000
1 to 5 years	783,621,000			783,621,000			1,006,221,000
5 to 10 years	631,276,000			631,276,000			574,207,000
Over 10 years	922,676,000			922,676,000			832,704,000
Available-for-sale securities, debt maturities, fair value	2,693,329,000			2,693,329,000			2,730,199,000
Gross investment gains on sale of investments	10,800,000		4,100,000	15,100,000		5,800,000
Gross investment losses on sale of investments	100,000		0	100,000		100,000
State and municipal bonds [Member]
Available-for-sale securities [Abstract]
Amortized cost	1,073,585,000			1,073,585,000			970,746,000
Unrealized gain	68,231,000			68,231,000			62,215,000
Unrealized loss	(506,000)			(506,000)			(7,000)
Fair value	1,141,310,000			1,141,310,000			1,032,954,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	46,826,000			46,826,000			9,436,000
12 months or more	0			0			0
Total	46,826,000			46,826,000			9,436,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(506,000)			(506,000)			(7,000)
12 months or more	0			0			0
Total	(506,000)			(506,000)			(7,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	1,141,310,000			1,141,310,000			1,032,954,000
U.S. Treasury securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	79,706,000			79,706,000			88,934,000
Unrealized gain	1,184,000			1,184,000			2,410,000
Unrealized loss	(11,000)			(11,000)			(4,000)
Fair value	80,879,000			80,879,000			91,340,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	18,635,000			18,635,000			4,932,000
12 months or more	0			0			0
Total	18,635,000			18,635,000			4,932,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(11,000)			(11,000)			(4,000)
12 months or more	0			0			0
Total	(11,000)			(11,000)			(4,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	80,879,000			80,879,000			91,340,000
Government-sponsored enterprise securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	23,417,000	[2]		23,417,000	[2]		140,595,000	[2]
Unrealized gain	1,115,000	[2]		1,115,000	[2]		2,694,000	[2]
Unrealized loss	0	[2]		0	[2]		(11,000)	[2]
Fair value	24,532,000	[2]		24,532,000	[2]		143,278,000	[2]
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	0			0			12,495,000
12 months or more	0			0			0
Total	0			0			12,495,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	0			0			(11,000)
12 months or more	0			0			0
Total	0			0			(11,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	24,532,000	[2]		24,532,000	[2]		143,278,000	[2]
Residential mortgage-backed securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	335,891,000	[3]		335,891,000	[3]		354,713,000	[3]
Unrealized gain	15,113,000	[3]		15,113,000	[3]		14,097,000	[3]
Unrealized loss	(123,000)	[3]		(123,000)	[3]		(12,000)	[3]
Fair value	350,881,000	[3]		350,881,000	[3]		368,798,000	[3]
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	11,606,000			11,606,000			5,127,000
12 months or more	52,000			52,000			43,000
Total	11,658,000			11,658,000			5,170,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(122,000)			(122,000)			(11,000)
12 months or more	(1,000)			(1,000)			(1,000)
Total	(123,000)			(123,000)			(12,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	350,881,000	[3]		350,881,000	[3]		368,798,000	[3]
Commercial mortgage-backed securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	12,772,000			12,772,000			13,801,000
Unrealized gain	1,163,000			1,163,000			1,024,000
Unrealized loss	0			0			0
Fair value	13,935,000			13,935,000			14,825,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	0			0			0
12 months or more	0			0			0
Total	0			0			0
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	0			0			0
12 months or more	0			0			0
Total	0			0			0
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	13,935,000			13,935,000			14,825,000
Asset Backed Securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	9,566,000	[4]		9,566,000	[4]		12,840,000	[4]
Unrealized gain	373,000	[4]		373,000	[4]		664,000	[4]
Unrealized loss	0	[4]		0	[4]		0	[4]
Fair value	9,939,000	[4]		9,939,000	[4]		13,504,000	[4]
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	0			0			0
12 months or more	0			0			0
Total	0			0			0
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	0			0			0
12 months or more	0			0			0
Total	0			0			0
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	9,939,000	[4]		9,939,000	[4]		13,504,000	[4]
Corporate debt and other securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	1,050,565,000			1,050,565,000			1,051,874,000
Unrealized gain	23,230,000			23,230,000			23,804,000
Unrealized loss	(1,942,000)			(1,942,000)			(10,178,000)
Fair value	1,071,853,000			1,071,853,000			1,065,500,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	86,740,000			86,740,000			350,294,000
12 months or more	101,017,000			101,017,000			0
Total	187,757,000			187,757,000			350,294,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(599,000)			(599,000)			(10,178,000)
12 months or more	(1,343,000)			(1,343,000)			0
Total	(1,942,000)			(1,942,000)			(10,178,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	$ 1,071,853,000			$ 1,071,853,000			$ 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, 2011. As of June 30, 2012, the Company no longer held any TLGP securities.

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

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