INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 124,665	$ 177,756
Less than 12 Months Unrealized Loss	(761)	(967)
12 Months or More Fair Value	339,320	496,104
12 Months or More Unrealized Loss	(10,178)	(16,090)
Total Fair Value	463,985	673,860
Total Unrealized Loss	(10,939)	(17,057)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	0	493
Less than 12 Months Unrealized Loss	0	(1)
12 Months or More Fair Value	0	97
12 Months or More Unrealized Loss	0	0
Total Fair Value	0	590
Total Unrealized Loss	0	(1)
Mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	27,187	119,641
Less than 12 Months Unrealized Loss	(47)	(420)
12 Months or More Fair Value	299,124	428,486
12 Months or More Unrealized Loss	(8,545)	(13,780)
Total Fair Value	326,311	548,127
Total Unrealized Loss	(8,592)	(14,200)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	21,688	12,746
Less than 12 Months Unrealized Loss	(214)	(126)
12 Months or More Fair Value	23,393	37,516
12 Months or More Unrealized Loss	(1,054)	(1,014)
Total Fair Value	45,081	50,262
Total Unrealized Loss	(1,268)	(1,140)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	59,696	32,045
Less than 12 Months Unrealized Loss	(203)	(103)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	59,696	32,045
Total Unrealized Loss	(203)	(103)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	16,094	12,831
Less than 12 Months Unrealized Loss	(297)	(317)
12 Months or More Fair Value	16,803	30,005
12 Months or More Unrealized Loss	(579)	(1,296)
Total Fair Value	32,897	42,836
Total Unrealized Loss	$ (876)	$ (1,613)