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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.

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