FAIR VALUE MEASUREMENTS (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
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	419,022	418,654
Total assets at fair value	419,022	418,654
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	1,545,530	1,440,052
Total assets at fair value	1,545,530	1,440,052
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	1,964,552	1,858,706
Total assets at fair value	1,964,552	1,858,706
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	6,516	10,298
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	6,516	10,298
Corporate Bond Securities [Member] | 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	584,156	526,038
Corporate Bond Securities [Member] | Total | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	584,156	526,038
Mortgage-backed | 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	260,902	244,416
Mortgage-backed | Total | Fair value measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	260,902	244,416
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	138,118	106,309
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	138,118	106,309
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	10,557	13,678
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	10,557	13,678
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	32,163	17,303
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	32,163	17,303
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	513,118	522,010
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	513,118	522,010
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	419,022	418,654
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	$ 419,022	$ 418,654