LOANS, LEASES & ALLOWANCE FOR LOAN AND LEASE LOSSES - PERFORMING AND NONACCRUAL BY CLASS AND SOURCE (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jun. 30, 2017
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	$ 8,517,686	$ 7,449,261
Performing
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	8,486,460	7,420,868
Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	31,226	28,393
Mortgage
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	4,198,941	3,901,754
Mortgage | Performing
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	4,170,495	3,878,377
Mortgage | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	28,446	23,377
Mortgage | Origination
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	4,159,204	3,846,969
Mortgage | Origination | Performing
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	4,134,011	3,827,649
Mortgage | Origination | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	25,193	19,320
Mortgage | Purchased
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	39,737	54,785
Mortgage | Purchased | Performing
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	36,484	50,728
Mortgage | Purchased | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	3,253	4,057
Multifamily real estate secured
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	1,800,919	1,619,404
Multifamily real estate secured | Performing
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	1,800,687	1,615,149
Multifamily real estate secured | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	232	4,255
Multifamily real estate secured | Origination
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	1,735,051	1,532,890
Multifamily real estate secured | Origination | Performing
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	1,735,051	1,528,912
Multifamily real estate secured | Origination | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	0	3,978
Multifamily real estate secured | Purchased
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	65,868	86,514
Multifamily real estate secured | Purchased | Performing
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	65,636	86,237
Multifamily real estate secured | Purchased | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	232	277
Commercial real estate secured
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	220,379	162,715
Commercial real estate secured | Performing
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	220,379	162,715
Commercial real estate secured | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	0	0
Commercial real estate secured | Origination
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	212,235	150,880
Commercial real estate secured | Origination | Performing
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	212,235	150,880
Commercial real estate secured | Origination | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	0	0
Commercial real estate secured | Purchased
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	8,144	11,835
Commercial real estate secured | Purchased | Performing
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	8,144	11,835
Commercial real estate secured | Purchased | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Gross loans and leases	$ 0	$ 0