AVAILABLE-FOR-SALE INVESTMENT SECURITIES (Details 2) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value
Less Than 12 Months	$ 2,527,519	$ 414,390
12 Months or More	228,733	718,223
Total	2,756,252	1,132,613
Gross Unrealized Losses
Less Than 12 Months	(16,075)	(2,051)
12 Months or More	(6,272)	(11,917)
Total	(22,347)	(13,968)
U.S. Treasury securities
Fair Value
Less Than 12 Months	907,400	170,260
12 Months or More	20,282	163,800
Total	927,682	334,060
Gross Unrealized Losses
Less Than 12 Months	(4,250)	(266)
12 Months or More	(142)	(1,371)
Total	(4,392)	(1,637)
U.S. government agency and U.S. government sponsored enterprise debt securities
Fair Value
Less Than 12 Months	541,385	69,438
12 Months or More	0	124,104
Total	541,385	193,542
Gross Unrealized Losses
Less Than 12 Months	(2,994)	(504)
12 Months or More	0	(889)
Total	(2,994)	(1,393)
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	252,340	45,405
12 Months or More	20,793	16,169
Total	273,133	61,574
Gross Unrealized Losses
Less Than 12 Months	(2,562)	(257)
12 Months or More	(462)	(336)
Total	(3,024)	(593)
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Residential mortgage-backed securities
Fair Value
Less Than 12 Months	535,842	81,927
12 Months or More	58,315	241,047
Total	594,157	322,974
Gross Unrealized Losses
Less Than 12 Months	(4,530)	(270)
12 Months or More	(1,871)	(3,808)
Total	(6,401)	(4,078)
Municipal securities
Fair Value
Less Than 12 Months	48,495	6,391
12 Months or More	14,739	61,107
Total	63,234	67,498
Gross Unrealized Losses
Less Than 12 Months	(437)	(26)
12 Months or More	(356)	(1,136)
Total	(793)	(1,162)
Other residential mortgage-backed securities: | Investment grade
Fair Value
Less Than 12 Months	5,123	0
12 Months or More	6,242	7,217
Total	11,365	7,217
Gross Unrealized Losses
Less Than 12 Months	(1)	0
12 Months or More	(172)	(135)
Total	(173)	(135)
Corporate debt securities: | Investment grade
Fair Value
Less Than 12 Months	218,944	25,084
12 Months or More	89,989	90,098
Total	308,933	115,182
Gross Unrealized Losses
Less Than 12 Months	(1,189)	(12)
12 Months or More	(1,151)	(1,042)
Total	(2,340)	(1,054)
Corporate debt securities: | Non-investment grade
Fair Value
Less Than 12 Months	0	0
12 Months or More	9,642	14,681
Total	9,642	14,681
Gross Unrealized Losses
Less Than 12 Months	0	0
12 Months or More	(1,849)	(3,200)
Total	(1,849)	(3,200)
Other securities
Fair Value
Less Than 12 Months	17,990	15,885
12 Months or More	8,731	0
Total	26,721	15,885
Gross Unrealized Losses
Less Than 12 Months	(112)	(716)
12 Months or More	(269)	0
Total	$ (381)	$ (716)