AVAILABLE-FOR-SALE INVESTMENT SECURITIES (Details) - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Investment Securities
Amortized Cost		$ 2,972,935	$ 2,619,205
Gross Unrealized Gains		20,161	21,128
Gross Unrealized Losses		(10,950)	(13,968)
Fair Value		2,982,146	2,626,365
U.S. Treasury securities
Schedule of Available-for-sale Investment Securities
Amortized Cost		1,194,790	873,101
Gross Unrealized Gains		5,493	1,971
Gross Unrealized Losses		(172)	(1,637)
Fair Value		1,200,111	873,435
U.S. government agency and U.S. government sponsored enterprise debt securities
Schedule of Available-for-sale Investment Securities
Amortized Cost		425,495	311,927
Gross Unrealized Gains		584	490
Gross Unrealized Losses		(1,216)	(1,393)
Fair Value		424,863	311,024
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities
Schedule of Available-for-sale Investment Securities
Amortized Cost		187,831	140,957
Gross Unrealized Gains		502	1,056
Gross Unrealized Losses		(945)	(593)
Fair Value		187,388	141,420
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Residential mortgage-backed securities
Schedule of Available-for-sale Investment Securities
Amortized Cost		682,740	785,412
Gross Unrealized Gains		9,043	9,754
Gross Unrealized Losses		(3,071)	(4,078)
Fair Value		688,712	791,088
Municipal securities
Schedule of Available-for-sale Investment Securities
Amortized Cost		205,703	245,408
Gross Unrealized Gains		3,580	6,202
Gross Unrealized Losses		(2,861)	(1,162)
Fair Value		206,422	250,448
Other residential mortgage-backed securities | Investment grade
Schedule of Available-for-sale Investment Securities
Amortized Cost	[1]	61,737	52,694
Gross Unrealized Gains	[1]	466	1,359
Gross Unrealized Losses	[1]	(295)	(135)
Fair Value	[1]	61,908	53,918
Other commercial mortgage-backed securities | Investment grade
Schedule of Available-for-sale Investment Securities
Amortized Cost	[1]		34,000
Gross Unrealized Gains	[1]		53
Fair Value	[1]		34,053
Corporate debt securities | Investment grade
Schedule of Available-for-sale Investment Securities
Amortized Cost	[1]	157,200	116,236
Gross Unrealized Gains	[1]	336
Gross Unrealized Losses	[1]	(558)	(1,054)
Fair Value	[1]	156,978	115,182
Corporate debt securities | Non-investment grade
Schedule of Available-for-sale Investment Securities
Amortized Cost	[1]	11,525	17,881
Gross Unrealized Losses	[1]	(1,500)	(3,200)
Fair Value	[1]	10,025	14,681
Other securities
Schedule of Available-for-sale Investment Securities
Amortized Cost		45,914	41,589
Gross Unrealized Gains		157	243
Gross Unrealized Losses		(332)	(716)
Fair Value		$ 45,739	$ 41,116

[1] Available-for-sale investment securities rated BBB- or higher by S&P or Baa3 or higher by Moody's are considered investment grade. Conversely, available-for-sale investment securities rated lower than BBB- by S&P or lower than Baa3 by Moody's are considered non-investment grade.