RELATED-PARTY TRANSACTIONS - Transactions Involving Underwriting Services (Details) - USD ($)	Oct. 27, 2016	May 11, 2016	Mar. 08, 2016	Feb. 08, 2016	Apr. 08, 2015	Apr. 06, 2015	May 21, 2014	Sep. 30, 2016
Related Party Transaction [Line Items]
Expenses from transactions with related parties						$ 263,000
Expenses from transactions with related parties, net of related expenses						221,000
Preferred stock, liquidation preference per share (in dollars per share)								$ 1,000
Senior Notes | 5.25% Senior Notes Due April 15, 2025
Related Party Transaction [Line Items]
Aggregate principal amount issued						$ 175,000,000
Stated interest rate						5.25%
Series E 7.00% non-cumulative perpetual
Related Party Transaction [Line Items]
Depository shares to preferred stock ratio				2.50%
Non cumulative preferred stock, dividend rate				7.00%
Preferred stock, liquidation preference per share (in dollars per share)				$ 1,000
Offering price per share (in dollars per share)				$ 25
Series D 7.375% non-cumulative perpetual
Related Party Transaction [Line Items]
Depository shares to preferred stock ratio					2.50%
Non cumulative preferred stock, dividend rate					7.375%
Preferred stock, liquidation preference per share (in dollars per share)					$ 1,000
Offering price per share (in dollars per share)					$ 25
Director | Financial Advisory Services | Subsequent Event
Related Party Transaction [Line Items]
Expenses from transactions with related parties	$ 516,000
Expenses from transactions with related parties, net of related expenses	$ 500,000
Director | Underwriting Fees
Related Party Transaction [Line Items]
Expenses from transactions with related parties				$ 944,000	$ 590,000
Expenses from transactions with related parties, net of related expenses				$ 849,000	$ 515,000
Common Stock
Related Party Transaction [Line Items]
Issuance of stock shares		5,250,000	4,850,000
Common Stock | Director | Underwriting Fees
Related Party Transaction [Line Items]
Expenses from transactions with related parties			$ 1,000,000				$ 521,000
Expenses from transactions with related parties, net of related expenses			$ 846,000				$ 481,000
Underwritten Public Offering
Related Party Transaction [Line Items]
Depository shares issued (in shares)				5,000,000	4,600,000
Offering price per share (in dollars per share)				$ 25	$ 25
Underwritten Public Offering | Series E 7.00% non-cumulative perpetual
Related Party Transaction [Line Items]
Depository shares issued (in shares)				5,000,000
Depository shares to preferred stock ratio				2.50%
Preferred stock, liquidation preference per share (in dollars per share)				$ 1,000
Underwritten Public Offering | Series D 7.375% non-cumulative perpetual
Related Party Transaction [Line Items]
Depository shares to preferred stock ratio					2.50%
Preferred stock, liquidation preference per share (in dollars per share)					$ 1,000
Underwritten Public Offering | Common Stock
Related Party Transaction [Line Items]
Issuance of stock shares			5,577,500				5,922,500