DERIVATIVE INSTRUMENTS - Summary of Characteristics of Interest Rate Swaps (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Interest Rate Swaps
Derivative Instruments:
Current Notional	$ 65,971,950,000	$ 35,329,200,000
Weighted Average Pay Rate	2.00%	2.22%
Weighted Average Receive Rate	2.13%	1.58%
Weighted Average Years to Maturity	4 years 6 months 7 days	6 years 8 months 19 days
Interest Rate Swaps | Weighted Average
Derivative Instruments:
Fixed rate on forward starting pay swaps		1.86%
Interest Rate Swaps | 0 - 3 years
Derivative Instruments:
Derivative Instruments minimum maturity period	0 years	0 years
Derivative Instruments maximum maturity period	3 years	3 years
Current Notional	$ 38,344,000,000	$ 6,532,000,000
Weighted Average Pay Rate	1.64%	1.56%
Weighted Average Receive Rate	2.19%	1.62%
Weighted Average Years to Maturity	1 year 9 months 8 days	2 years 29 days
Interest Rate Swaps | 3 - 6 years
Derivative Instruments:
Derivative Instruments minimum maturity period	3 years	3 years
Derivative Instruments maximum maturity period	6 years	6 years
Current Notional	$ 13,254,550,000	$ 14,791,800,000
Weighted Average Pay Rate	2.17%	2.12%
Weighted Average Receive Rate	2.12%	1.57%
Weighted Average Years to Maturity	4 years 8 months 1 day	4 years 6 months 4 days
Interest Rate Swaps | 6 - 10 years
Derivative Instruments:
Derivative Instruments minimum maturity period	6 years	6 years
Derivative Instruments maximum maturity period	10 years	10 years
Current Notional	$ 10,547,000,000	$ 10,179,000,000
Weighted Average Pay Rate	2.32%	2.35%
Weighted Average Receive Rate	2.02%	1.58%
Weighted Average Years to Maturity	8 years 1 month 9 days	8 years 15 days
Interest Rate Swaps | Greater than 10 years
Derivative Instruments:
Derivative Instruments minimum maturity period	10 years	10 years
Current Notional	$ 3,826,400,000	$ 3,826,400,000
Weighted Average Pay Rate	3.65%	3.65%
Weighted Average Receive Rate	1.94%	1.51%
Weighted Average Years to Maturity	18 years 2 months 20 days	18 years 5 months 19 days
Forward Starting Pay Fixed Swaps
Derivative Instruments:
Current Notional	$ 0	$ 8,100,000,000