DERIVATIVE INSTRUMENTS - Summary of Characteristics of Interest Rate Swaps (Details) - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Interest rate swaps
Derivative [Line Items]
Current Notional	$ 67,093,700,000	$ 67,093,700,000	$ 70,465,700,000
Weighted Average Pay Rate	2.12%	2.12%	2.17%
Weighted Average Receive Rate	2.46%	2.46%	2.68%
Weighted Average Underlying Years to Maturity		4 years 7 months 17 days	4 years 3 months 3 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	$ 34,092,400,000	$ 34,092,400,000	$ 31,900,200,000
Weighted Average Pay Rate	1.82%	1.82%	1.84%
Weighted Average Receive Rate	2.44%	2.44%	2.73%
Weighted Average Underlying Years to Maturity		1 year 4 months 13 days	1 year 2 months 15 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	$ 12,300,800,000	$ 12,300,800,000	$ 16,603,200,000
Weighted Average Pay Rate	2.00%	2.00%	2.29%
Weighted Average Receive Rate	2.47%	2.47%	2.70%
Weighted Average Underlying Years to Maturity		4 years 5 months 12 days	4 years 3 months 18 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	$ 17,122,500,000	$ 17,122,500,000	$ 18,060,900,000
Weighted Average Pay Rate	2.52%	2.52%	2.57%
Weighted Average Receive Rate	2.51%	2.51%	2.56%
Weighted Average Underlying Years to Maturity		8 years 9 months 14 days	8 years 7 months 13 days
Interest rate swaps | Greater than 10 years
Derivative [Line Items]
Derivative Instruments minimum maturity period	10 years		10 years
Current Notional	$ 3,578,000,000	$ 3,578,000,000	$ 3,901,400,000
Weighted Average Pay Rate	3.59%	3.59%	3.63%
Weighted Average Receive Rate	2.44%	2.44%	2.59%
Weighted Average Underlying Years to Maturity		17 years 6 months 21 days	17 years 3 months 29 days
Credit derivatives
Derivative [Line Items]
Purchase contracts	$ 30,000,000.0	$ 30,000,000.0	$ 30,000,000
Current Notional	336,000,000.0	336,000,000.0	451,000,000.0
Forward Starting Pay Fixed Swaps
Derivative [Line Items]
Current Notional	$ 0	$ 0	$ 0