BANK LOANS, NET, Allowance for Loan Losses, Loans Individually and Collectively Evaluated for Impairment (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014
Allowance for loan losses:
Individually evaluated for impairment	$ 5,919		$ 5,178
Collectively evaluated for impairment	188,301		167,079
Total allowance for loan losses	194,220	$ 185,459	172,257	$ 160,008	$ 156,767	$ 147,574
Recorded investment:
Individually evaluated for impairment	74,291		78,101
Collectively evaluated for impairment	14,335,629		12,995,082
Total loans held for investment	14,409,920		13,073,183
Reserve for unfunded lending commitments [Abstract]
Reserve for unfunded lending commitments	7,800		9,700
C&I loans
Allowance for loan losses:
Total allowance for loan losses	137,299		117,623
Recorded investment:
Total loans held for investment	7,283,214		6,928,018
CRE construction
Allowance for loan losses:
Total allowance for loan losses	2,553		2,707
Recorded investment:
Total loans held for investment	145,905		162,356
CRE
Allowance for loan losses:
Total allowance for loan losses	32,668		30,486
Recorded investment:
Total loans held for investment	2,448,268		2,054,154
Tax-exempt loans
Allowance for loan losses:
Total allowance for loan losses	7,034		5,949
Recorded investment:
Total loans held for investment	610,274		484,537
Residential mortgage
Allowance for loan losses:
Total allowance for loan losses	11,254		12,526
Recorded investment:
Total loans held for investment	2,217,584		1,962,614
SBL
Allowance for loan losses:
Total allowance for loan losses	3,412		2,966
Recorded investment:
Total loans held for investment	1,704,675		1,481,504
Loans held for investment | C&I loans
Allowance for loan losses:
Individually evaluated for impairment	3,105		1,132
Collectively evaluated for impairment	134,194		116,491
Total allowance for loan losses	137,299	128,721	117,623	111,125	109,582	103,179
Recorded investment:
Individually evaluated for impairment	11,391		10,599
Collectively evaluated for impairment	7,271,823		6,917,419
Loans held for investment | CRE construction
Allowance for loan losses:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	2,553		2,707
Total allowance for loan losses	2,553	2,635	2,707	1,675	1,709	1,594
Recorded investment:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	145,905		162,356
Loans held for investment | CRE
Allowance for loan losses:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	32,668		30,486
Total allowance for loan losses	32,668	31,304	30,486	25,717	25,095	25,022
Recorded investment:
Individually evaluated for impairment	4,497		4,796
Collectively evaluated for impairment	2,443,771		2,049,358
Loans held for investment | Tax-exempt loans
Allowance for loan losses:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	7,034		5,949
Total allowance for loan losses	7,034	7,119	5,949	3,909	2,738	1,380
Recorded investment:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	610,274		484,537
Loans held for investment | Residential mortgage
Allowance for loan losses:
Individually evaluated for impairment	2,814		4,046
Collectively evaluated for impairment	8,440		8,480
Total allowance for loan losses	11,254	12,265	12,526	15,076	15,319	14,350
Recorded investment:
Individually evaluated for impairment	58,403		62,706
Collectively evaluated for impairment	2,159,181		1,899,908
Loans held for investment | SBL
Allowance for loan losses:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	3,412		2,966
Total allowance for loan losses	3,412	$ 3,415	2,966	$ 2,506	$ 2,324	$ 2,049
Recorded investment:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	$ 1,704,675		$ 1,481,504