DERIVATIVE INSTRUMENTS - Summary of Characteristics of Interest Rate Swaps (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Interest Rate Swaps
Derivative Instruments:
Current Notional	$ 64,089,750,000	$ 35,329,200,000
Weighted Average Pay Rate	2.08%	2.22%
Weighted Average Receive Rate	2.31%	1.58%
Weighted Average Years to Maturity	4 years 5 months 5 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	$ 32,086,800,000	$ 6,532,000,000
Weighted Average Pay Rate	1.76%	1.56%
Weighted Average Receive Rate	2.34%	1.62%
Weighted Average Years to Maturity	1 year 7 months 12 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	$ 15,449,650,000	$ 14,791,800,000
Weighted Average Pay Rate	2.27%	2.12%
Weighted Average Receive Rate	2.31%	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	$ 12,476,900,000	$ 10,179,000,000
Weighted Average Pay Rate	2.45%	2.35%
Weighted Average Receive Rate	2.24%	1.58%
Weighted Average Years to Maturity	8 years 6 months 32 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	$ 4,076,400,000	$ 3,826,400,000
Weighted Average Pay Rate	3.58%	3.65%
Weighted Average Receive Rate	2.21%	1.51%
Weighted Average Years to Maturity	17 years 4 months 52 days	18 years 5 months 19 days
Forward Starting Pay Fixed Swaps
Derivative Instruments:
Current Notional	$ 0	$ 8,100,000,000