INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 843,564	$ 1,209,292
Less than 12 Months Unrealized Loss	(19,802)	(41,454)
12 Months or More Fair Value	306,652	118,662
12 Months or More Unrealized Loss	(17,762)	(6,708)
Total Fair Value	1,150,216	1,327,954
Total Unrealized Loss	(37,564)	(48,162)
Securities of U.S. government agencies and corporations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	27,887	27,851
Less than 12 Months Unrealized Loss	(564)	(970)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	27,887	27,851
Total Unrealized Loss	(564)	(970)
Mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	677,538	966,718
Less than 12 Months Unrealized Loss	(15,392)	(32,432)
12 Months or More Fair Value	281,627	108,929
12 Months or More Unrealized Loss	(15,271)	(6,101)
Total Fair Value	959,165	1,075,647
Total Unrealized Loss	(30,663)	(38,533)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	50,100	66,502
Less than 12 Months Unrealized Loss	(2,440)	(5,294)
12 Months or More Fair Value	17,248	1,935
12 Months or More Unrealized Loss	(1,982)	(46)
Total Fair Value	67,348	68,437
Total Unrealized Loss	(4,422)	(5,340)
Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	48,937	93,355
Less than 12 Months Unrealized Loss	(100)	(616)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Loss	0	0
Total Fair Value	48,937	93,355
Total Unrealized Loss	(100)	(616)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	39,102	54,866
Less than 12 Months Unrealized Loss	(1,306)	(2,142)
12 Months or More Fair Value	7,777	7,798
12 Months or More Unrealized Loss	(509)	(561)
Total Fair Value	46,879	62,664
Total Unrealized Loss	$ (1,815)	$ (2,703)