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VARIABLE INTEREST ENTITIES - Summary of Unconsolidated VIEs (Details)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
investment
Variable Interest Entity [Line Items]          
Funded commitments $ 0 $ 15,000,000 $ (1,027,000) $ 45,926,000  
Gain (loss) on investments in alternative energy partnerships (2,484,000) (8,348,000) (4,258,000) (26,791,000)  
Tax credit 412,000 8,800,000 9,600,000 33,300,000  
Number of investments with completed funding | investment         1
Number of investments | investment         2
Aggregate funding commitment 100,000,000   100,000,000   $ 100,000,000
Total amount funded 49,900,000   49,900,000   62,800,000
Canceled commitment 50,100,000   50,100,000   37,200,000
Amount unused and returned 1,000,000   1,000,000    
Funding of equity investment     2,874,000 24,506,000  
Affordable Housing Fund Investment          
Variable Interest Entity [Line Items]          
Aggregate funding commitment 29,300,000   29,300,000    
Total amount funded 15,600,000   15,600,000    
Funding of equity investment 384,000 2,200,000 2,000,000 4,200,000  
Amortization expense 498,000 255,000 1,500,000 982,000  
Alternative investments unfunded investment commitments 13,600,000   13,600,000    
Variable Interest Entity, Not Primary Beneficiary          
Variable Interest Entity [Line Items]          
Gain (loss) on investments in alternative energy partnerships 2,500,000 8,300,000 4,300,000 26,800,000  
Tax credit 412,000 8,800,000 9,600,000 33,300,000  
Cash 6,281,000   6,281,000   16,518,000
Equipment, net of depreciation 261,304,000   261,304,000   246,297,000
Other assets 3,952,000   3,952,000   2,444,000
Total unconsolidated assets 271,537,000   271,537,000   265,259,000
Total unconsolidated liabilities 6,127,000   6,127,000   7,181,000
Maximum loss exposure 41,781,000   41,781,000   98,910,000
Variable Interest Entity, Not Primary Beneficiary | Affordable Housing Fund Investment          
Variable Interest Entity [Line Items]          
Tax credit 470,000 98,000 1,400,000 637,000  
Maximum loss exposure 20,500,000   20,500,000   $ 22,000,000
Variable Interest Entity, Not Primary Beneficiary | Other Assets          
Variable Interest Entity [Line Items]          
Total unconsolidated assets $ 41,800,000 $ 43,800,000 $ 41,800,000 $ 43,800,000