DERIVATIVE INSTRUMENTS - Summary of Characteristics of Interest Rate Swaps (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Interest rate swaps
Derivative [Line Items]
Current Notional	$ 35,329,200,000	$ 25,875,665,000
Weighted Average Pay Rate	2.22%	2.22%
Weighted Average Receive Rate	1.58%	1.02%
Weighted Average Underlying Years to Maturity	6 years 8 months 19 days	6 years 10 months 13 days
Interest rate swaps | 0 - 3 years
Derivative [Line Items]
Derivative Instruments minimum maturity period	0 years	0 years
Derivative Instruments maximum maturity period	3 years	3 years
Current Notional	$ 6,532,000,000	$ 3,444,365,000
Weighted Average Pay Rate	1.56%	1.37%
Weighted Average Receive Rate	1.62%	1.00%
Weighted Average Underlying Years to Maturity	2 years 29 days	2 years 8 months 16 days
Interest rate swaps | 3 - 6 years
Derivative [Line Items]
Derivative Instruments minimum maturity period	3 years	3 years
Derivative Instruments maximum maturity period	6 years	6 years
Current Notional	$ 14,791,800,000	$ 10,590,000,000
Weighted Average Pay Rate	2.12%	1.92%
Weighted Average Receive Rate	1.57%	0.99%
Weighted Average Underlying Years to Maturity	4 years 6 months 4 days	3 years 11 months 9 days
Interest rate swaps | 6 - 10 years
Derivative [Line Items]
Derivative Instruments minimum maturity period	6 years	6 years
Derivative Instruments maximum maturity period	10 years	10 years
Current Notional	$ 10,179,000,000	$ 8,206,900,000
Weighted Average Pay Rate	2.35%	2.35%
Weighted Average Receive Rate	1.58%	1.10%
Weighted Average Underlying Years to Maturity	8 years 15 days	7 years 9 months 26 days
Interest rate swaps | Greater than 10 years
Derivative [Line Items]
Derivative Instruments minimum maturity period	10 years	10 years
Current Notional	$ 3,826,400,000	$ 3,634,400,000
Weighted Average Pay Rate	3.65%	3.70%
Weighted Average Receive Rate	1.51%	0.83%
Weighted Average Underlying Years to Maturity	18 years 5 months 19 days	18 years 4 months 10 days
Forward Starting Pay Fixed Swaps
Derivative [Line Items]
Current Notional	$ 8,100,000,000	$ 0
Weighted Average | Interest rate swaps
Derivative [Line Items]
Fixed rate on forward starting pay swaps	1.86%