INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 1,080,204	$ 280,435
Less than 12 Months Unrealized Loss	(26,138)	(1,895)
12 Months or More Fair Value	25,119	25,513
12 Months or More Unrealized Loss	(350)	(405)
Total Fair Value	1,105,323	305,948
Total Unrealized Loss	(26,488)	(2,300)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	19,625
Less than 12 Months Unrealized Loss	(225)
12 Months or More Fair Value	0
12 Months or More Unrealized Loss	0
Total Fair Value	19,625
Total Unrealized Loss	(225)
Mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	865,450	240,641
Less than 12 Months Unrealized Loss	(21,140)	(1,635)
12 Months or More Fair Value	23,867	25,513
12 Months or More Unrealized Loss	(231)	(405)
Total Fair Value	889,317	266,154
Total Unrealized Loss	(21,371)	(2,040)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	68,190	21,341
Less than 12 Months Unrealized Loss	(2,648)	(96)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	68,190	21,341
Total Unrealized Loss	(2,648)	(96)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	60,486	9,999
Less than 12 Months Unrealized Loss	(388)	(1)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	60,486	9,999
Total Unrealized Loss	(388)	(1)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	66,453	8,454
Less than 12 Months Unrealized Loss	(1,737)	(163)
12 Months or More Fair Value	1,252	0
12 Months or More Unrealized Loss	(119)	0
Total Fair Value	67,705	8,454
Total Unrealized Loss	$ (1,856)	$ (163)