FAIR VALUE, Significant Assumptions Used in Valuation of Level 3 Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Mar. 31, 2015	Sep. 30, 2014
Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Total assets at fair value on a recurring basis	2,215,029	$ 1,995,685
Nonrecurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Total assets at fair value on a nonrecurring basis	132,336	113,706
Significant unobservable inputs (level 3) | Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Total assets at fair value on a recurring basis	426,922	417,283
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Recent trades
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Total assets at fair value on a recurring basis	61,500
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Recent trades | Minimum
Fair Value Inputs [Abstract]
Observed trades (in inactive markets) of in-portfolio securities	94.00%
Fair Value Inputs, Comparability Adjustments	(3.00%)
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Recent trades | Maximum
Fair Value Inputs [Abstract]
Observed trades (in inactive markets) of in-portfolio securities	94.00%
Fair Value Inputs, Comparability Adjustments	3.00%
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Recent trades | Weighted average
Fair Value Inputs [Abstract]
Observed trades (in inactive markets) of in-portfolio securities	94.00%
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Probability weighted internal scenario models
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Total assets at fair value on a recurring basis	10,505
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Probability weighted internal scenario 1 | Minimum
Fair Value Inputs [Abstract]
Observed trades (in inactive markets) of in-portfolio securities	70.00%
Weighting assigned to outcome of scenario 1 / scenario 2	20.00%
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Probability weighted internal scenario 1 | Maximum
Fair Value Inputs [Abstract]
Observed trades (in inactive markets) of in-portfolio securities	70.00%
Weighting assigned to outcome of scenario 1 / scenario 2	80.00%
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Probability weighted internal scenario 1 | Weighted average
Fair Value Inputs [Abstract]
Observed trades (in inactive markets) of in-portfolio securities	70.00%
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Probability weighted internal scenario 2 | Minimum
Fair Value Inputs [Abstract]
Average discount rate	5.10%
Average interest rates applicable to future interest income on the securities	1.49%
Prepayment year	Dec. 31, 2017
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Probability weighted internal scenario 2 | Maximum
Fair Value Inputs [Abstract]
Average discount rate	6.94%
Average interest rates applicable to future interest income on the securities	3.19%
Prepayment year	Dec. 31, 2024
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Probability weighted internal scenario 2 | Weighted average
Fair Value Inputs [Abstract]
Average discount rate	6.02%
Average interest rates applicable to future interest income on the securities	2.34%
Prepayment year	Dec. 31, 2021
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Discounted cash flow
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Total assets at fair value on a recurring basis	17,609
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Discounted cash flow | Minimum
Fair Value Inputs [Abstract]
Average discount rate	3.32%
Average interest rates applicable to future interest income on the securities	1.28%
Prepayment year	Dec. 31, 2017
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Discounted cash flow | Maximum
Fair Value Inputs [Abstract]
Average discount rate	5.94%
Average interest rates applicable to future interest income on the securities	4.25%
Prepayment year	Dec. 31, 2024
Significant unobservable inputs (level 3) | Recurring | ARS - municipals | Discounted cash flow | Weighted average
Fair Value Inputs [Abstract]
Average discount rate	3.84%
Average interest rates applicable to future interest income on the securities	1.41%
Prepayment year	Dec. 31, 2020
Significant unobservable inputs (level 3) | Recurring | ARS - preferred securities | Discounted cash flow
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Total assets at fair value on a recurring basis	112,448
Significant unobservable inputs (level 3) | Recurring | ARS - preferred securities | Discounted cash flow | Minimum
Fair Value Inputs [Abstract]
Average discount rate	3.43%
Average interest rates applicable to future interest income on the securities	1.88%
Prepayment year	Dec. 31, 2015
Significant unobservable inputs (level 3) | Recurring | ARS - preferred securities | Discounted cash flow | Maximum
Fair Value Inputs [Abstract]
Average discount rate	4.91%
Average interest rates applicable to future interest income on the securities	3.21%
Prepayment year	Dec. 31, 2019
Significant unobservable inputs (level 3) | Recurring | ARS - preferred securities | Discounted cash flow | Weighted average
Fair Value Inputs [Abstract]
Average discount rate	4.13%
Average interest rates applicable to future interest income on the securities	1.99%
Prepayment year	Dec. 31, 2019
Significant unobservable inputs (level 3) | Recurring | Private equity investments | Probability weighted internal scenario 1 | Minimum
Fair Value Inputs [Abstract]
Weighting assigned to outcome of scenario 1 / scenario 2	72.00%
Significant unobservable inputs (level 3) | Recurring | Private equity investments | Probability weighted internal scenario 1 | Maximum
Fair Value Inputs [Abstract]
Weighting assigned to outcome of scenario 1 / scenario 2	28.00%
Significant unobservable inputs (level 3) | Recurring | Private equity investments | Discounted cash flow
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Total assets at fair value on a recurring basis	46,402
Significant unobservable inputs (level 3) | Recurring | Private equity investments | Discounted cash flow | Minimum
Fair Value Inputs [Abstract]
Discount rate	13.00%
Terminal growth rate of cash flows	3.00%
Terminal year	Dec. 31, 2016
Significant unobservable inputs (level 3) | Recurring | Private equity investments | Discounted cash flow | Maximum
Fair Value Inputs [Abstract]
Discount rate	17.50%
Terminal growth rate of cash flows	3.00%
Terminal year	Dec. 31, 2018
Significant unobservable inputs (level 3) | Recurring | Private equity investments | Discounted cash flow | Weighted average
Fair Value Inputs [Abstract]
Discount rate	15.90%
Terminal growth rate of cash flows	3.00%
Terminal year	Dec. 31, 2017
Significant unobservable inputs (level 3) | Recurring | Private equity investments | Scenario 2 - market approach - market multiple method | Minimum
Fair Value Inputs [Abstract]
EBITDA multiple	4.75
Significant unobservable inputs (level 3) | Recurring | Private equity investments | Scenario 2 - market approach - market multiple method | Maximum
Fair Value Inputs [Abstract]
EBITDA multiple	7.50
Significant unobservable inputs (level 3) | Recurring | Private equity investments | Scenario 2 - market approach - market multiple method | Weighted average
Fair Value Inputs [Abstract]
EBITDA multiple	6.30
Significant unobservable inputs (level 3) | Recurring | Private equity investments | Transaction price or other investment-specific events
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Total assets at fair value on a recurring basis	174,542
Significant unobservable inputs (level 3) | Recurring | Impaired loans residential | Discounted cash flow | Minimum
Fair Value Inputs [Abstract]
Prepayment rate (in years)	7 years
Significant unobservable inputs (level 3) | Recurring | Impaired loans residential | Discounted cash flow | Maximum
Fair Value Inputs [Abstract]
Prepayment rate (in years)	12 years
Significant unobservable inputs (level 3) | Recurring | Impaired loans residential | Discounted cash flow | Weighted average
Fair Value Inputs [Abstract]
Prepayment rate (in years)	10 years 3 months 18 days
Significant unobservable inputs (level 3) | Nonrecurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Total assets at fair value on a nonrecurring basis	51,444	55,528
Significant unobservable inputs (level 3) | Nonrecurring | Impaired loans residential | Discounted cash flow | Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Total assets at fair value on a nonrecurring basis	24,075
Significant unobservable inputs (level 3) | Nonrecurring | Impaired loans corporate | Appraisal, discounted cash flow, or distressed enterprise value
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Total assets at fair value on a nonrecurring basis	27,369