INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 600,677	$ 743,530
Less than 12 Months Unrealized Loss	(6,836)	(11,565)
12 Months or More Fair Value	163,323	243,220
12 Months or More Unrealized Loss	(3,653)	(4,533)
Total Fair Value	764,000	986,750
Total Unrealized Loss	(10,489)	(16,098)
Residential Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	201,854	180,654
Less than 12 Months Unrealized Loss	(3,077)	(3,621)
12 Months or More Fair Value	9,021	9,890
12 Months or More Unrealized Loss	(280)	(285)
Total Fair Value	210,875	190,544
Total Unrealized Loss	(3,357)	(3,906)
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)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	6,281	19,917
Less than 12 Months Unrealized Loss	(17)	(150)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	6,281	19,917
Total Unrealized Loss	(17)	(150)
Commercial Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	94,052	123,122
Less than 12 Months Unrealized Loss	(1,059)	(1,200)
12 Months or More Fair Value	60,816	65,007
12 Months or More Unrealized Loss	(1,265)	(1,443)
Total Fair Value	154,868	188,129
Total Unrealized Loss	(2,324)	(2,643)
Collateralized Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	193,840	201,305
Less than 12 Months Unrealized Loss	(1,454)	(2,882)
12 Months or More Fair Value	45,729	42,314
12 Months or More Unrealized Loss	(1,238)	(915)
Total Fair Value	239,569	243,619
Total Unrealized Loss	(2,692)	(3,797)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	79,823	94,632
Less than 12 Months Unrealized Loss	(1,006)	(2,710)
12 Months or More Fair Value	14,664	12,023
12 Months or More Unrealized Loss	(282)	(150)
Total Fair Value	94,487	106,655
Total Unrealized Loss	(1,288)	(2,860)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	14,147	116,057
Less than 12 Months Unrealized Loss	(89)	(764)
12 Months or More Fair Value	30,621	92,629
12 Months or More Unrealized Loss	(415)	(1,249)
Total Fair Value	44,768	208,686
Total Unrealized Loss	(504)	(2,013)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	10,680	7,746
Less than 12 Months Unrealized Loss	(134)	(237)
12 Months or More Fair Value	2,472	21,357
12 Months or More Unrealized Loss	(173)	(491)
Total Fair Value	13,152	29,103
Total Unrealized Loss	$ (307)	$ (728)