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INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value $ 495,192 $ 961,572
Less than 12 Months Unrealized Loss (2,527) (6,345)
12 Months or More Fair Value 759,792 882,865
12 Months or More Unrealized Loss (15,693) (23,557)
Total Fair Value 1,254,984 1,844,437
Total Unrealized Loss (18,220) (29,902)
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 98 97
12 Months or More Unrealized Loss (1) (2)
Total Fair Value 98 97
Total Unrealized Loss (1) (2)
Securities of U.S. government agencies and corporations    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 6,046 0
Less than 12 Months Unrealized Loss (77) 0
12 Months or More Fair Value 13,946 16,777
12 Months or More Unrealized Loss (278) (233)
Total Fair Value 19,992 16,777
Total Unrealized Loss (355) (233)
Residential Mortgage Backed Securities [Member]    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 12,585 186,029
Less than 12 Months Unrealized Loss (322) (935)
12 Months or More Fair Value 239,305 264,795
12 Months or More Unrealized Loss (3,798) (7,273)
Total Fair Value 251,890 450,824
Total Unrealized Loss (4,120) (8,208)
Commercial Mortgage Backed Securities [Member]    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 140,670 147,754
Less than 12 Months Unrealized Loss (264) (369)
12 Months or More Fair Value 195,282 232,363
12 Months or More Unrealized Loss (7,138) (7,597)
Total Fair Value 335,952 380,117
Total Unrealized Loss (7,402) (7,966)
Collateralized Mortgage Backed Securities [Member]    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 90,610 194,795
Less than 12 Months Unrealized Loss (431) (1,546)
12 Months or More Fair Value 212,286 240,514
12 Months or More Unrealized Loss (3,065) (5,245)
Total Fair Value 302,896 435,309
Total Unrealized Loss (3,496) (6,791)
Obligations of state and other political subdivisions    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 12,080 62,805
Less than 12 Months Unrealized Loss (30) (299)
12 Months or More Fair Value 42,954 86,644
12 Months or More Unrealized Loss (702) (2,489)
Total Fair Value 55,034 149,449
Total Unrealized Loss (732) (2,788)
Asset-backed Securities [Member]    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 224,565 336,437
Less than 12 Months Unrealized Loss (1,325) (2,312)
12 Months or More Fair Value 48,376 37,105
12 Months or More Unrealized Loss (465) (498)
Total Fair Value 272,941 373,542
Total Unrealized Loss (1,790) (2,810)
Other securities    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 8,636 33,752
Less than 12 Months Unrealized Loss (78) (884)
12 Months or More Fair Value 7,545 4,570
12 Months or More Unrealized Loss (246) (220)
Total Fair Value 16,181 38,322
Total Unrealized Loss $ (324) $ (1,104)