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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Investments, Unrealized Loss Position [Line Items]      
Proceeds from Sale of Held-to-maturity Securities $ 0 $ 4,862 $ 0
Less than 12 Months Fair Value 570,095 743,530  
Less than 12 Months Unrealized Loss (4,745) (11,565)  
12 Months or More Fair Value 335,516 243,220  
12 Months or More Unrealized Loss (8,881) (4,533)  
Total Fair Value 905,611 986,750  
Total Unrealized Loss (13,626) (16,098)  
U.S. Treasuries      
Investments, Unrealized Loss Position [Line Items]      
Less than 12 Months Fair Value 97 97  
Less than 12 Months Unrealized Loss (1) (1)  
12 Months or More Fair Value 0 0  
12 Months or More Unrealized Loss 0 0  
Total Fair Value 97 97  
Total Unrealized Loss (1) (1)  
Securities of U.S. government agencies and corporations      
Investments, Unrealized Loss Position [Line Items]      
Less than 12 Months Fair Value 11,092 19,917  
Less than 12 Months Unrealized Loss (76) (150)  
12 Months or More Fair Value 0 0  
12 Months or More Unrealized Loss 0 0  
Total Fair Value 11,092 19,917  
Total Unrealized Loss (76) (150)  
Residential Mortgage Backed Securities [Member]      
Investments, Unrealized Loss Position [Line Items]      
Less than 12 Months Fair Value 175,183 180,654  
Less than 12 Months Unrealized Loss (1,109) (3,621)  
12 Months or More Fair Value 108,782 9,890  
12 Months or More Unrealized Loss (3,025) (285)  
Total Fair Value 283,965 190,544  
Total Unrealized Loss (4,134) (3,906)  
Commercial Mortgage Backed Securities [Member]      
Investments, Unrealized Loss Position [Line Items]      
Less than 12 Months Fair Value 132,818 123,122  
Less than 12 Months Unrealized Loss (1,713) (1,200)  
12 Months or More Fair Value 72,139 65,007  
12 Months or More Unrealized Loss (2,704) (1,443)  
Total Fair Value 204,957 188,129  
Total Unrealized Loss (4,417) (2,643)  
Collateralized Mortgage Backed Securities [Member]      
Investments, Unrealized Loss Position [Line Items]      
Less than 12 Months Fair Value 164,909 201,305  
Less than 12 Months Unrealized Loss (1,138) (2,882)  
12 Months or More Fair Value 101,436 42,314  
12 Months or More Unrealized Loss (2,325) (915)  
Total Fair Value 266,345 243,619  
Total Unrealized Loss (3,463) (3,797)  
Obligations of state and other political subdivisions      
Investments, Unrealized Loss Position [Line Items]      
Less than 12 Months Fair Value 38,450 94,632  
Less than 12 Months Unrealized Loss (507) (2,710)  
12 Months or More Fair Value 21,639 12,023  
12 Months or More Unrealized Loss (435) (150)  
Total Fair Value 60,089 106,655  
Total Unrealized Loss (942) (2,860)  
Asset-backed Securities [Member]      
Investments, Unrealized Loss Position [Line Items]      
Less than 12 Months Fair Value 44,941 116,057  
Less than 12 Months Unrealized Loss (200) (764)  
12 Months or More Fair Value 24,396 92,629  
12 Months or More Unrealized Loss (106) (1,249)  
Total Fair Value 69,337 208,686  
Total Unrealized Loss (306) (2,013)  
Other securities      
Investments, Unrealized Loss Position [Line Items]      
Less than 12 Months Fair Value 2,605 7,746  
Less than 12 Months Unrealized Loss (1) (237)  
12 Months or More Fair Value 7,124 21,357  
12 Months or More Unrealized Loss (286) (491)  
Total Fair Value 9,729 29,103  
Total Unrealized Loss $ (287) $ (728)