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INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value $ 1,838,375 $ 570,095
Less than 12 Months Unrealized Loss (21,924) (4,745)
12 Months or More Fair Value 370,966 335,516
12 Months or More Unrealized Loss (17,045) (8,881)
Total Fair Value 2,209,341 905,611
Total Unrealized Loss (38,969) (13,626)
U.S. Treasuries    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 95 97
Less than 12 Months Unrealized Loss (3) (1)
12 Months or More Fair Value 0 0
12 Months or More Unrealized Loss 0 0
Total Fair Value 95 97
Total Unrealized Loss (3) (1)
Securities of U.S. government agencies and corporations    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 19,890 11,092
Less than 12 Months Unrealized Loss (311) (76)
12 Months or More Fair Value 5,379 0
12 Months or More Unrealized Loss (141) 0
Total Fair Value 25,269 11,092
Total Unrealized Loss (452) (76)
Residential Mortgage Backed Securities [Member]    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 488,725 175,183
Less than 12 Months Unrealized Loss (5,650) (1,109)
12 Months or More Fair Value 129,218 108,782
12 Months or More Unrealized Loss (6,017) (3,025)
Total Fair Value 617,943 283,965
Total Unrealized Loss (11,667) (4,134)
Commercial Mortgage Backed Securities [Member]    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 278,366 132,818
Less than 12 Months Unrealized Loss (5,414) (1,713)
12 Months or More Fair Value 88,624 72,139
12 Months or More Unrealized Loss (5,148) (2,704)
Total Fair Value 366,990 204,957
Total Unrealized Loss (10,562) (4,417)
Collateralized Mortgage Backed Securities [Member]    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 628,667 164,909
Less than 12 Months Unrealized Loss (6,910) (1,138)
12 Months or More Fair Value 99,142 101,436
12 Months or More Unrealized Loss (4,078) (2,325)
Total Fair Value 727,809 266,345
Total Unrealized Loss (10,988) (3,463)
Obligations of state and other political subdivisions    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 186,269 38,450
Less than 12 Months Unrealized Loss (2,195) (507)
12 Months or More Fair Value 27,441 21,639
12 Months or More Unrealized Loss (1,143) (435)
Total Fair Value 213,710 60,089
Total Unrealized Loss (3,338) (942)
Asset-backed Securities [Member]    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 219,064 44,941
Less than 12 Months Unrealized Loss (1,215) (200)
12 Months or More Fair Value 13,591 24,396
12 Months or More Unrealized Loss (139) (106)
Total Fair Value 232,655 69,337
Total Unrealized Loss (1,354) (306)
Other securities    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 17,299 2,605
Less than 12 Months Unrealized Loss (226) (1)
12 Months or More Fair Value 7,571 7,124
12 Months or More Unrealized Loss (379) (286)
Total Fair Value 24,870 9,729
Total Unrealized Loss $ (605) $ (287)