INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 412,554	$ 177,756
Less than 12 Months Unrealized Loss	(3,180)	(967)
12 Months or More Fair Value	324,866	496,104
12 Months or More Unrealized Loss	(8,411)	(16,090)
Total Fair Value	737,420	673,860
Total Unrealized Loss	(11,591)	(17,057)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		493
Less than 12 Months Unrealized Loss		(1)
12 Months or More Fair Value		97
12 Months or More Unrealized Loss		0
Total Fair Value		590
Total Unrealized Loss		(1)
Mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	216,209	119,641
Less than 12 Months Unrealized Loss	(856)	(420)
12 Months or More Fair Value	257,294	428,486
12 Months or More Unrealized Loss	(6,543)	(13,780)
Total Fair Value	473,503	548,127
Total Unrealized Loss	(7,399)	(14,200)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,268	12,746
Less than 12 Months Unrealized Loss	(3)	(126)
12 Months or More Fair Value	36,309	37,516
12 Months or More Unrealized Loss	(1,242)	(1,014)
Total Fair Value	37,577	50,262
Total Unrealized Loss	(1,245)	(1,140)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	157,585	32,045
Less than 12 Months Unrealized Loss	(1,781)	(103)
12 Months or More Fair Value	5,780	0
12 Months or More Unrealized Loss	(20)	0
Total Fair Value	163,365	32,045
Total Unrealized Loss	(1,801)	(103)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	37,492	12,831
Less than 12 Months Unrealized Loss	(540)	(317)
12 Months or More Fair Value	25,483	30,005
12 Months or More Unrealized Loss	(606)	(1,296)
Total Fair Value	62,975	42,836
Total Unrealized Loss	$ (1,146)	$ (1,613)