INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Investments, Unrealized Loss Position [Line Items]
Document Period End Date	Mar. 31, 2020
Less than 12 Months Fair Value	$ 701,681	$ 481,608
Less than 12 Months Unrealized Loss	(25,910)	(1,814)
12 Months or More Fair Value	135,893	214,322
12 Months or More Unrealized Loss	(9,124)	(4,493)
Total Fair Value	837,574	695,930
Total Unrealized Loss	(35,034)	(6,307)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	46,746	0
Less than 12 Months Unrealized Loss	(3,081)	0
12 Months or More Fair Value	4,010	5,649
12 Months or More Unrealized Loss	(805)	(115)
Total Fair Value	50,756	5,649
Total Unrealized Loss	(3,886)	(115)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	279,980	125,889
Less than 12 Months Unrealized Loss	(16,048)	(553)
12 Months or More Fair Value	59,852	54,963
12 Months or More Unrealized Loss	(2,793)	(511)
Total Fair Value	339,832	180,852
Total Unrealized Loss	(18,841)	(1,064)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	33,083	118,623
Less than 12 Months Unrealized Loss	(148)	(457)
12 Months or More Fair Value	1,681	7,950
12 Months or More Unrealized Loss	(4)	(36)
Total Fair Value	34,764	126,573
Total Unrealized Loss	(152)	(493)
Collateralized Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	115,697	85,248
Less than 12 Months Unrealized Loss	(1,847)	(297)
12 Months or More Fair Value	8,267	30,628
12 Months or More Unrealized Loss	(632)	(196)
Total Fair Value	123,964	115,876
Total Unrealized Loss	(2,479)	(493)
Commercial Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	188,990	111,658
Less than 12 Months Unrealized Loss	(4,567)	(298)
12 Months or More Fair Value	62,083	104,069
12 Months or More Unrealized Loss	(4,890)	(3,571)
Total Fair Value	251,073	215,727
Total Unrealized Loss	(9,457)	(3,869)
Residential Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	37,185	40,190
Less than 12 Months Unrealized Loss	(219)	(209)
12 Months or More Fair Value	0	11,063
12 Months or More Unrealized Loss	0	(64)
Total Fair Value	37,185	51,253
Total Unrealized Loss	(219)	(273)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	0	0
Less than 12 Months Unrealized Loss	0	0
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	0	0
Total Unrealized Loss	0	0
U.S. Treasuries
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	0	0
Less than 12 Months Unrealized Loss	0	0
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	0	0
Total Unrealized Loss	$ 0	$ 0