PARENT-ONLY CONDENSED FINANCIAL INFORMATION - STATEMENT OF CASH FLOWS (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 45,300	$ 56,057	$ 41,295	$ 40,786	$ 40,634	$ 38,821	$ 38,835	$ 36,841	$ 183,438	$ 155,131	$ 152,411
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Accretion of discounts on securities									291	(264)	(624)
Amortization of borrowing costs									208	208	208
Net gain on investment securities									0	(709)	18
Stock-based compensation expense									21,935	23,439	20,399
Decrease (increase) in other assets									(36,979)	(27,564)	(47,070)
Net cash provided by operating activities									284,118	204,421	167,915
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from sale of available-for-sale securities									0	15,863	52,714
Origination of loans and leases held for investment									(6,573,568)	(6,756,832)	(5,895,902)
Purchases of loans and leases, net of discounts and premiums									0	(11,525)	0
Proceeds from principal repayments on loans									5,349,800	5,846,349	4,818,558
Net cash provided by (used in) investing activities									(1,348,439)	(931,741)	(1,026,005)
CASH FLOWS FROM FINANCING ACTIVITIES:
Tax payments related to settlement of restricted stock units									(7,457)	(9,916)	(9,952)
Repurchase of treasury stock									(38,858)	(56,437)	(35,183)
Proceeds from issuance of subordinated notes									0	7,400	0
Cash dividends on preferred stock									(386)	(232)	(309)
Net cash provided by (used in) financing activities									2,157,472	961,838	837,399
NET CHANGE IN CASH AND CASH EQUIVALENTS									1,093,151	234,518	(20,691)
CASH AND CASH EQUIVALENTS—Beginning of year				857,368				622,850	857,368	622,850	643,541
CASH AND CASH EQUIVALENTS—End of year	1,950,519				857,368				1,950,519	857,368	622,850
Parent
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income									183,438	155,131	152,411
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Accretion of discounts on securities									26	0	(2)
Amortization of borrowing costs									208	208	208
Net gain on investment securities									0	0	(153)
Stock-based compensation expense									21,935	23,439	20,399
Equity in undistributed earnings of subsidiary									(79,898)	(93,733)	(97,452)
Decrease (increase) in other assets									(72,268)	(8,477)	(4,938)
Increase (decrease) in other liabilities									74,295	7,986	5,528
Net cash provided by operating activities									127,736	84,554	76,001
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of investment securities									(15,301)	0	0
Proceeds from sale of available-for-sale securities									0	0	162
Origination of loans and leases held for investment									0	(844)	0
Purchases of loans and leases, net of discounts and premiums									(59,391)	0	0
Proceeds from principal repayments on loans									10	854	9
Investment in subsidiary									(10,130)	(106,557)	(4,000)
Net cash provided by (used in) investing activities									(84,812)	(106,547)	(3,829)
CASH FLOWS FROM FINANCING ACTIVITIES:
Tax effect from vesting of restricted stock units									0	0	7
Tax payments related to settlement of restricted stock units									(7,457)	(9,916)	(9,958)
Repurchase of treasury stock									(38,858)	(56,437)	(35,183)
Proceeds from issuance of subordinated notes									0	7,400	0
Cash dividends on preferred stock									(386)	(232)	(309)
Net cash provided by (used in) financing activities									(46,701)	(59,185)	(45,443)
NET CHANGE IN CASH AND CASH EQUIVALENTS									(3,777)	(81,178)	26,729
CASH AND CASH EQUIVALENTS—Beginning of year				$ 26,907				$ 108,085	26,907	108,085	81,356
CASH AND CASH EQUIVALENTS—End of year	$ 23,130				$ 26,907				$ 23,130	$ 26,907	$ 108,085