FAIR VALUE MEASUREMENTS (Details) - USD ($) $ in Thousands	Jun. 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
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	391,152	$ 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	1,537,365	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
Total | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,928,517	1,905,729
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	30,400	33,788
U.S. government | Total | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	30,400	33,788
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	13,644	6,747
U.S. Agency | Total | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	13,644	6,747
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	1,840	10,665
Non-U.S. govt. & agency | Total | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,840	10,665
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	239,180	264,468
Agency MBS | Total | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	239,180	264,468
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	88,299	135,304
ABS/CMBS | Total | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	88,299	135,304
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	601,960	562,690
Corporate. | Total | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	601,960	562,690
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	562,042	481,425
Municipal | Total | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	562,042	481,425
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	391,152	410,642
Equity securities | Total | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	$ 391,152	$ 410,642