FAIR VALUE MEASUREMENTS (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers in (out of) level 1	$ 0
Transfers in (out of) level 2	0
Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,901,351	$ 1,905,729
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	369,047	410,642
Significant Other Observable Inputs (Level 2) | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,532,304	1,495,087
Significant Unobservable Inputs (Level 3) | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0
U.S. government | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	33,036	33,788
U.S. government | Significant Other Observable Inputs (Level 2) | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	33,036	33,788
U.S. Agency | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	13,867	6,747
U.S. Agency | Significant Other Observable Inputs (Level 2) | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	13,867	6,747
Non-U.S. govt. & agency | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	4,664	10,665
Non-U.S. govt. & agency | Significant Other Observable Inputs (Level 2) | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	4,664	10,665
Agency MBS | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	261,272	264,468
Agency MBS | Significant Other Observable Inputs (Level 2) | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	261,272	264,468
ABS/CMBS | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	84,809	135,304
ABS/CMBS | Significant Other Observable Inputs (Level 2) | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	84,809	135,304
Corporate. | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	602,023	562,690
Corporate. | Significant Other Observable Inputs (Level 2) | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	602,023	562,690
Municipal | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	532,633	481,425
Municipal | Significant Other Observable Inputs (Level 2) | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	532,633	481,425
Equity securities | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	369,047	410,642
Equity securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	$ 369,047	$ 410,642