INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 288,460	$ 1,209,292
Less than 12 Months Unrealized Loss	(2,889)	(41,454)
12 Months or More Fair Value	583,216	118,662
12 Months or More Unrealized Loss	(22,830)	(6,708)
Total Fair Value	871,676	1,327,954
Total Unrealized Loss	(25,719)	(48,162)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,470	27,851
Less than 12 Months Unrealized Loss	(5)	(970)
12 Months or More Fair Value	17,944	0
12 Months or More Unrealized Loss	(126)	0
Total Fair Value	19,414	27,851
Total Unrealized Loss	(131)	(970)
Mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	252,959	966,718
Less than 12 Months Unrealized Loss	(2,616)	(32,432)
12 Months or More Fair Value	496,254	108,929
12 Months or More Unrealized Loss	(20,077)	(6,101)
Total Fair Value	749,213	1,075,647
Total Unrealized Loss	(22,693)	(38,533)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	14,267	66,502
Less than 12 Months Unrealized Loss	(174)	(5,294)
12 Months or More Fair Value	38,578	1,935
12 Months or More Unrealized Loss	(1,323)	(46)
Total Fair Value	52,845	68,437
Total Unrealized Loss	(1,497)	(5,340)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	10,510	93,355
Less than 12 Months Unrealized Loss	(1)	(616)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	10,510	93,355
Total Unrealized Loss	(1)	(616)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	9,254	54,866
Less than 12 Months Unrealized Loss	(93)	(2,142)
12 Months or More Fair Value	30,440	7,798
12 Months or More Unrealized Loss	(1,304)	(561)
Total Fair Value	39,694	62,664
Total Unrealized Loss	$ (1,397)	$ (2,703)