INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 1,032,629	$ 280,435
Less than 12 Months Unrealized Loss	(32,770)	(1,895)
12 Months or More Fair Value	42,923	25,513
12 Months or More Unrealized Loss	(390)	(405)
Total Fair Value	1,075,552	305,948
Total Unrealized Loss	(33,160)	(2,300)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	28,806
Less than 12 Months Unrealized Loss	(756)
12 Months or More Fair Value	0
12 Months or More Unrealized Loss	0
Total Fair Value	28,806
Total Unrealized Loss	(756)
Mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	807,599	240,641
Less than 12 Months Unrealized Loss	(24,767)	(1,635)
12 Months or More Fair Value	41,679	25,513
12 Months or More Unrealized Loss	(261)	(405)
Total Fair Value	849,278	266,154
Total Unrealized Loss	(25,028)	(2,040)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	64,895	21,341
Less than 12 Months Unrealized Loss	(4,357)	(96)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	64,895	21,341
Total Unrealized Loss	(4,357)	(96)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	59,003	9,999
Less than 12 Months Unrealized Loss	(359)	(1)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	59,003	9,999
Total Unrealized Loss	(359)	(1)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	72,326	8,454
Less than 12 Months Unrealized Loss	(2,531)	(163)
12 Months or More Fair Value	1,244	0
12 Months or More Unrealized Loss	(129)	0
Total Fair Value	73,570	8,454
Total Unrealized Loss	$ (2,660)	$ (163)