INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 402,835	$ 280,435
Less than 12 Months Unrealized Loss	(3,860)	(1,895)
12 Months or More Fair Value	25,867	25,513
12 Months or More Unrealized Loss	(436)	(405)
Total Fair Value	428,702	305,948
Total Unrealized Loss	(4,296)	(2,300)
Mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	356,139	240,641
Less than 12 Months Unrealized Loss	(3,547)	(1,635)
12 Months or More Fair Value	25,022	25,513
12 Months or More Unrealized Loss	(366)	(405)
Total Fair Value	381,161	266,154
Total Unrealized Loss	(3,913)	(2,040)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	25,604	21,341
Less than 12 Months Unrealized Loss	(202)	(96)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	25,604	21,341
Total Unrealized Loss	(202)	(96)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		9,999
Less than 12 Months Unrealized Loss		(1)
12 Months or More Fair Value		0
12 Months or More Unrealized Loss		0
Total Fair Value		9,999
Total Unrealized Loss		(1)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	21,092	8,454
Less than 12 Months Unrealized Loss	(111)	(163)
12 Months or More Fair Value	845	0
12 Months or More Unrealized Loss	(70)	0
Total Fair Value	21,937	8,454
Total Unrealized Loss	$ (181)	$ (163)