FAIR VALUE - Securities in Unrealized Loss Position (Details) - USD ($) $ in Thousands |
3 Months Ended | 12 Months Ended |
|---|---|---|
Mar. 31, 2018 |
Dec. 31, 2017 |
|
| Securities in an unrealized loss position, fair value [Abstract] | ||
| One year or greater | $ 2,342,577 | $ 2,552,668 |
| Less than one year | 4,844,667 | 4,665,883 |
| Fair Value, Total | 7,187,244 | 7,218,551 |
| Securities in an unrealized loss position, aggregate loss [Abstract] | ||
| One year or greater | 34,142 | 26,701 |
| Less than one year | 54,005 | 22,752 |
| Unrealized Losses, Total | $ 88,147 | $ 49,453 |