RELATED PARTY TRANSACTIONS - Schedule of Related Party Debt Terms (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Related Party Transaction [Line Items]
Long-term borrowed funds	$ 4,153	$ 4,642
Subordinated Debt | RBSG | 5.158% fixed-to-floating rate subordinated debt, (LIBOR 3.56%) callable, due 2023
Related Party Transaction [Line Items]
Interest rate	5.158%
Long-term borrowed funds	$ 333	333
Subordinated Debt | RBSG | 4.771% fixed rate subordinated debt, due 2023
Related Party Transaction [Line Items]
Interest rate	4.771%
Long-term borrowed funds	$ 333	333
Subordinated Debt | RBSG | 4.691% fixed rate subordinated debt, due 2024
Related Party Transaction [Line Items]
Interest rate	4.691%
Long-term borrowed funds	$ 334	334
Subordinated Debt | RBSG | 4.153% fixed rate subordinated debt due 2024
Related Party Transaction [Line Items]
Interest rate	4.153%
Long-term borrowed funds	$ 333	333
Subordinated Debt | RBSG | 4.023% fixed rate subordinated debt, due 2024
Related Party Transaction [Line Items]
Interest rate	4.023%
Long-term borrowed funds	$ 333	333
Subordinated Debt | RBS | 4.082% fixed rate subordinated debt, due 2025
Related Party Transaction [Line Items]
Interest rate	4.082%
Long-term borrowed funds	$ 334	$ 334