BANK LOANS, NET, Analysis of Payment Status of Loans Held for Investment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2015
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	$ 1,656		$ 1,656		$ 3,099
Nonaccrual	59,425		59,425		52,619
Current and accruing	14,348,839		14,348,839		13,017,465
Total loans held for investment	14,409,920		14,409,920		13,073,183
Performing nonaccrual loans	31,200		31,200		22,400
Impaired interest lost on nonaccrual loans	500	$ 500	800	$ 1,200
Impaired financing receivable, interest income recognized	200	$ 400	500	$ 600
Other assets
Analysis of payment status of loans held for investment [Abstract]
Other real estate owned	4,500		4,500		4,600
C&I loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	152		152		163
Nonaccrual	11,391		11,391		0
Current and accruing	7,271,671		7,271,671		6,927,855
Total loans held for investment	7,283,214		7,283,214		6,928,018
CRE construction loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
Nonaccrual	0		0		0
Current and accruing	145,905		145,905		162,356
Total loans held for investment	145,905		145,905		162,356
CRE loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
Nonaccrual	4,497		4,497		4,796
Current and accruing	2,443,771		2,443,771		2,049,358
Total loans held for investment	2,448,268		2,448,268		2,054,154
Tax-exempt loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
Nonaccrual	0		0		0
Current and accruing	610,274		610,274		484,537
Total loans held for investment	610,274		610,274		484,537
Residential - first mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	1,504		1,504		2,906
Nonaccrual	43,365		43,365		47,504
Current and accruing	2,153,501		2,153,501		1,891,384
Total loans held for investment	2,198,370		2,198,370		1,941,794
Residential mortgage - home equity loans/lines
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		30
Nonaccrual	172		172		319
Current and accruing	19,042		19,042		20,471
Total loans held for investment	19,214		19,214		20,820
SBL
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
Nonaccrual	0		0		0
Current and accruing	1,704,675		1,704,675		1,481,504
Total loans held for investment	1,704,675		1,704,675		1,481,504
Residential mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	2,217,584		2,217,584		1,962,614
30-89 days and accruing
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	1,656		1,656		3,099
30-89 days and accruing | C&I loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	152		152		163
30-89 days and accruing | CRE construction loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
30-89 days and accruing | CRE loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
30-89 days and accruing | Tax-exempt loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
30-89 days and accruing | Residential - first mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	1,504		1,504		2,906
30-89 days and accruing | Residential mortgage - home equity loans/lines
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		30
30-89 days and accruing | SBL
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
90 days or more and accruing
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
90 days or more and accruing | C&I loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
90 days or more and accruing | CRE construction loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
90 days or more and accruing | CRE loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
90 days or more and accruing | Tax-exempt loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
90 days or more and accruing | Residential - first mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
90 days or more and accruing | Residential mortgage - home equity loans/lines
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
90 days or more and accruing | SBL
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	0		0		0
One-to-four family residential mortgage loans | Residential mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	$ 21,600		$ 21,600		$ 24,600