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INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value $ 978,552 $ 570,095
Less than 12 Months Unrealized Loss (14,716) (4,745)
12 Months or More Fair Value 347,767 335,516
12 Months or More Unrealized Loss (14,779) (8,881)
Total Fair Value 1,326,319 905,611
Total Unrealized Loss (29,495) (13,626)
U.S. Treasuries    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 96 97
Less than 12 Months Unrealized Loss (2) (1)
12 Months or More Fair Value 0 0
12 Months or More Unrealized Loss 0 0
Total Fair Value 96 97
Total Unrealized Loss (2) (1)
Securities of U.S. government agencies and corporations    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 26,347 11,092
Less than 12 Months Unrealized Loss (381) (76)
12 Months or More Fair Value 0 0
12 Months or More Unrealized Loss 0 0
Total Fair Value 26,347 11,092
Total Unrealized Loss (381) (76)
Residential Mortgage Backed Securities [Member]    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 252,396 175,183
Less than 12 Months Unrealized Loss (3,712) (1,109)
12 Months or More Fair Value 123,671 108,782
12 Months or More Unrealized Loss (5,794) (3,025)
Total Fair Value 376,067 283,965
Total Unrealized Loss (9,506) (4,134)
Commercial Mortgage Backed Securities [Member]    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 233,201 132,818
Less than 12 Months Unrealized Loss (4,185) (1,713)
12 Months or More Fair Value 75,822 72,139
12 Months or More Unrealized Loss (4,038) (2,704)
Total Fair Value 309,023 204,957
Total Unrealized Loss (8,223) (4,417)
Collateralized Mortgage Backed Securities [Member]    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 274,159 164,909
Less than 12 Months Unrealized Loss (3,915) (1,138)
12 Months or More Fair Value 96,641 101,436
12 Months or More Unrealized Loss (3,674) (2,325)
Total Fair Value 370,800 266,345
Total Unrealized Loss (7,589) (3,463)
Obligations of state and other political subdivisions    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 98,207 38,450
Less than 12 Months Unrealized Loss (1,879) (507)
12 Months or More Fair Value 21,420 21,639
12 Months or More Unrealized Loss (789) (435)
Total Fair Value 119,627 60,089
Total Unrealized Loss (2,668) (942)
Asset-backed Securities [Member]    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 89,530 44,941
Less than 12 Months Unrealized Loss (613) (200)
12 Months or More Fair Value 23,113 24,396
12 Months or More Unrealized Loss (169) (106)
Total Fair Value 112,643 69,337
Total Unrealized Loss (782) (306)
Other securities    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 4,616 2,605
Less than 12 Months Unrealized Loss (29) (1)
12 Months or More Fair Value 7,100 7,124
12 Months or More Unrealized Loss (315) (286)
Total Fair Value 11,716 9,729
Total Unrealized Loss $ (344) $ (287)