INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 194,315	$ 726,559
Less than 12 Months Unrealized Loss	(1,980)	(9,062)
12 Months or More Fair Value	333,505	317,670
12 Months or More Unrealized Loss	(5,512)	(11,658)
Total Fair Value	527,820	1,044,229
Total Unrealized Loss	(7,492)	(20,720)
U.S. Treasuries
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		97
Less than 12 Months Unrealized Loss		(1)
12 Months or More Fair Value		0
12 Months or More Unrealized Loss		0
Total Fair Value		97
Total Unrealized Loss		(1)
Mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	102,227	500,768
Less than 12 Months Unrealized Loss	(531)	(5,362)
12 Months or More Fair Value	202,649	246,523
12 Months or More Unrealized Loss	(2,293)	(9,563)
Total Fair Value	304,876	747,291
Total Unrealized Loss	(2,824)	(14,925)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,369	5,800
Less than 12 Months Unrealized Loss	(11)	(65)
12 Months or More Fair Value	13,279	29,287
12 Months or More Unrealized Loss	(376)	(764)
Total Fair Value	14,648	35,087
Total Unrealized Loss	(387)	(829)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	75,261	189,066
Less than 12 Months Unrealized Loss	(895)	(3,042)
12 Months or More Fair Value	106,105	17,144
12 Months or More Unrealized Loss	(2,387)	(403)
Total Fair Value	181,366	206,210
Total Unrealized Loss	(3,282)	(3,445)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	15,458	30,828
Less than 12 Months Unrealized Loss	(543)	(592)
12 Months or More Fair Value	11,472	24,716
12 Months or More Unrealized Loss	(456)	(928)
Total Fair Value	26,930	55,544
Total Unrealized Loss	$ (999)	$ (1,520)