INVESTMENTS (Details) (USD $)	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012	Dec. 31, 2012
Available-for-sale securities [Abstract]
Amortized cost	$ 2,721,791,000			$ 2,644,753,000
Unrealized gain	90,344,000			111,913,000
Unrealized loss	(2,217,000)			(947,000)
Fair value	2,809,918,000			2,755,719,000
Fair value of equity method investments	24,118,000	[1]		24,605,000	[1]
Fair value of investments	2,834,036,000			2,780,324,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	375,565,000			204,949,000
12 months or more	214,000			1,738,000
Total	375,779,000			206,687,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(2,213,000)			(944,000)
12 months or more	(4,000)			(3,000)
Total	(2,217,000)			(947,000)
Available-for-sale securities, debt maturities, amortized cost basis [Abstract]
Within 1 year	468,436,000			362,116,000
1 to 5 years	725,945,000			798,143,000
5 to 10 years	711,930,000			652,941,000
Over 10 years	815,480,000			831,553,000
Total	2,721,791,000			2,644,753,000
Available-for-sale securities, debt maturities, fair value [Abstract]
Within 1 year	470,166,000			363,559,000
1 to 5 years	747,376,000			822,448,000
5 to 10 years	745,058,000			693,504,000
Over 10 years	847,318,000			876,208,000
Available-for-sale securities, debt maturities, fair value	2,809,918,000			2,755,719,000
Gross investment gains on sale of investments	12,800,000		4,300,000
Gross investment losses on sale of investments	300,000		0
State and municipal bonds [Member]
Available-for-sale securities [Abstract]
Amortized cost	1,120,913,000			1,176,016,000
Unrealized gain	66,748,000			78,272,000
Unrealized loss	(726,000)			(499,000)
Fair value	1,186,935,000			1,253,789,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	110,627,000			61,342,000
12 months or more	102,000			0
Total	110,729,000			61,342,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(725,000)			(499,000)
12 months or more	(1,000)			0
Total	(726,000)			(499,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	1,186,935,000			1,253,789,000
U.S. Treasury securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	114,448,000			78,264,000
Unrealized gain	577,000			669,000
Unrealized loss	(1,000)			(2,000)
Fair value	115,024,000			78,931,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	1,774,000			2,458,000
12 months or more	0			1,065,000
Total	1,774,000			3,523,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(1,000)			(1,000)
12 months or more	0			(1,000)
Total	(1,000)			(2,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	115,024,000			78,931,000
Government-sponsored enterprise securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	79,039,000			72,394,000
Unrealized gain	248,000			1,139,000
Unrealized loss	(2,000)			(1,000)
Fair value	79,285,000			73,532,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	11,786,000			15,714,000
12 months or more	0			0
Total	11,786,000			15,714,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(2,000)			(1,000)
12 months or more	0			0
Total	(2,000)			(1,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	79,285,000			73,532,000
Residential mortgage-backed securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	328,940,000	[2]		302,012,000	[2]
Unrealized gain	9,051,000	[2]		10,703,000	[2]
Unrealized loss	(793,000)	[2]		(74,000)	[2]
Fair value	337,198,000	[2]		312,641,000	[2]
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	120,987,000			23,861,000
12 months or more	60,000			59,000
Total	121,047,000			23,920,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(792,000)			(73,000)
12 months or more	(1,000)			(1,000)
Total	(793,000)			(74,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	337,198,000	[2]		312,641,000	[2]
Commercial mortgage-backed securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	33,938,000			21,416,000
Unrealized gain	80,000			193,000
Unrealized loss	(126,000)			(19,000)
Fair value	33,892,000			21,590,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	21,456,000			7,701,000
12 months or more	0			0
Total	21,456,000			7,701,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(126,000)			(19,000)
12 months or more	0			0
Total	(126,000)			(19,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	33,892,000			21,590,000
Asset Backed Securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	22,226,000	[3]		23,421,000	[3]
Unrealized gain	356,000	[3]		211,000	[3]
Unrealized loss	(10,000)	[3]		(6,000)	[3]
Fair value	22,572,000	[3]		23,626,000	[3]
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	13,988,000			14,492,000
12 months or more	0			0
Total	13,988,000			14,492,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(10,000)			(6,000)
12 months or more	0			0
Total	(10,000)			(6,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	22,572,000	[3]		23,626,000	[3]
Corporate debt and other securities [Member]
Available-for-sale securities [Abstract]
Amortized cost	1,022,287,000			971,230,000
Unrealized gain	13,284,000			20,726,000
Unrealized loss	(559,000)			(346,000)
Fair value	1,035,012,000			991,610,000
Continuous unrealized loss position, fair value [Abstract]
Less than 12 months	94,947,000			79,381,000
12 months or more	52,000			614,000
Total	94,999,000			79,995,000
Continuous unrealized loss position, aggregate losses [Abstract]
Less than 12 months	(557,000)			(345,000)
12 months or more	(2,000)			(1,000)
Total	(559,000)			(346,000)
Available-for-sale securities, debt maturities, fair value [Abstract]
Available-for-sale securities, debt maturities, fair value	$ 1,035,012,000			$ 991,610,000

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

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

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