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INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Investments, Unrealized Loss Position [Line Items]    
Document Period End Date Dec. 31, 2019  
Less than 12 Months Fair Value $ 481,608 $ 961,572
Less than 12 Months Unrealized Loss (1,814) (6,345)
12 Months or More Fair Value 214,322 882,865
12 Months or More Unrealized Loss (4,493) (23,557)
Total Fair Value 695,930 1,844,437
Total Unrealized Loss (6,307) (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 0 97
12 Months or More Unrealized Loss 0 (2)
Total Fair Value 0 97
Total Unrealized Loss 0 (2)
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 16,777
12 Months or More Unrealized Loss 0 (233)
Total Fair Value 0 16,777
Total Unrealized Loss 0 (233)
Mortgage-backed securities-residential    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 40,190 186,029
Less than 12 Months Unrealized Loss (209) (935)
12 Months or More Fair Value 11,063 264,795
12 Months or More Unrealized Loss (64) (7,273)
Total Fair Value 51,253 450,824
Total Unrealized Loss (273) (8,208)
Mortgage-backed securities-commercial    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 111,658 147,754
Less than 12 Months Unrealized Loss (298) (369)
12 Months or More Fair Value 104,069 232,363
12 Months or More Unrealized Loss (3,571) (7,597)
Total Fair Value 215,727 380,117
Total Unrealized Loss (3,869) (7,966)
Collateralized mortgage obligations    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 85,248 194,795
Less than 12 Months Unrealized Loss (297) (1,546)
12 Months or More Fair Value 30,628 240,514
12 Months or More Unrealized Loss (196) (5,245)
Total Fair Value 115,876 435,309
Total Unrealized Loss (493) (6,791)
Obligations of state and other political subdivisions    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 118,623 62,805
Less than 12 Months Unrealized Loss (457) (299)
12 Months or More Fair Value 7,950 86,644
12 Months or More Unrealized Loss (36) (2,489)
Total Fair Value 126,573 149,449
Total Unrealized Loss (493) (2,788)
Asset-backed Securities    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 125,889 336,437
Less than 12 Months Unrealized Loss (553) (2,312)
12 Months or More Fair Value 54,963 37,105
12 Months or More Unrealized Loss (511) (498)
Total Fair Value 180,852 373,542
Total Unrealized Loss (1,064) (2,810)
Other securities    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months Fair Value 0 33,752
Less than 12 Months Unrealized Loss 0 (884)
12 Months or More Fair Value 5,649 4,570
12 Months or More Unrealized Loss (115) (220)
Total Fair Value 5,649 38,322
Total Unrealized Loss $ (115) $ (1,104)