INVESTMENT SECURITIES (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Summary of Amortized Cost and Fair Value of Investment Securities

The following table presents the amortized cost and fair value of the investment securities portfolio as of the dates indicated:



($ in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
December 31, 2017
Securities available-for-sale:
SBA loan pool securities	$	1,056	$	2	$	—		$	1,058
U.S. government agency and U.S. government sponsored enterprise residential mortgage-backed securities	492,255		10		(15,336		)	476,929
Non-agency residential mortgage-backed securities	741		16		(1		)	756
Non-agency commercial mortgage-backed securities	305,172		5,339		—			310,511
Collateralized loan obligations	1,691,455		11,129		(266		)	1,702,318
Corporate debt securities	76,714		7,183		—			83,897
Total securities available-for-sale	$	2,567,393	$	23,679	$	(15,603	)	$	2,575,469
December 31, 2016
Securities held-to-maturity:
Non-agency commercial mortgage-backed securities	$	305,918	$	2,949	$	(1,781	)	$	307,086
Collateralized loan obligations	338,226		1,461		(61		)	339,626
Corporate debt securities	240,090		13,032		(91		)	253,031
Total securities held-to-maturity	$	884,234	$	17,442	$	(1,933	)	$	899,743
Securities available-for-sale:
SBA loan pool securities	$	1,221	$	—	$	—		$	1,221
U.S. government agency and U.S. government sponsored enterprise residential mortgage-backed securities	830,682		9		(23,418		)	807,273
Non-agency residential mortgage-backed securities	121,397		18		(4,238		)	117,177
Collateralized loan obligations	1,395,094		12,449		(674		)	1,406,869
Corporate debt securities	48,574		482		(108		)	48,948
Total securities available-for-sale	$	2,396,968	$	12,958	$	(28,438	)	$	2,381,488

Summary of Amortized Cost and Fair Value of Available-for-Sale Investment Securities Portfolio

The following table presents amortized cost and fair value of the available-for-sale investment securities portfolio by expected maturity. In the case of mortgage-backed securities, collateralized loan obligations, and SBA loan pool securities, expected maturities may differ from contractual maturities because borrowers generally have the right to call or prepay obligations with or without call or prepayment penalties. For that reason, mortgage-backed securities, collateralized loan obligations, and SBA loan pool securities are not included in the maturity categories.



	December 31, 2017
($ in thousands)	Amortized Cost		Fair Value
Maturity:
Within one year	$	—	$	—
One to five years	—		—
Five to ten years	76,714		83,897
Greater than ten years	—		—
Mortgage-backed securities, collateralized loan obligations, and SBA loan pool securities	2,490,679		2,491,572
Total	$	2,567,393	$	2,575,469

Proceeds from Sales and Calls of Securities and Associated Gross Gains and Losses

The following table presents proceeds from sales and calls of securities available-for-sale and the associated gross gains and losses realized through earnings upon the sales and calls of securities available-for-sale for the periods indicated:



	Year Ended December 31,
($ in thousands)	2017		2016			2015
Gross realized gains on sales and calls of securities available-for-sale	$	14,768	$	30,919		$	3,260
Gross realized losses on sales and calls of securities available-for-sale	—		(1,514		)	(2		)
Net realized gains on sales and calls of securities available-for-sale	$	14,768	$	29,405		$	3,258
Proceeds from sales and calls of securities available-for-sale	$	1,500,459	$	4,148,003		$	989,786
Tax expense on sales and calls of securities available-for-sale	$	6,180	$	12,218		$	1,368

Summary of Investment Securities with Unrealized Losses

The following table summarizes the investment securities with unrealized losses by security type and length of time in a continuous unrealized loss position as of the dates indicated:



	Less Than 12 Months					12 Months or Longer					Total
($ in thousands)	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
December 31, 2017
Securities available-for-sale:
U.S. government agency and U.S. government sponsored enterprise residential mortgage-backed securities	$	4,880	$	(35	)	$	470,092	$	(15,301	)	$	474,972	$	(15,336	)
Non-agency residential mortgage-backed securities	—		—			148		(1		)	148		(1		)
Collateralized loan obligations	104,334		(266		)	—		—			104,334		(266		)
Total securities available-for-sale	$	109,214	$	(301	)	$	470,240	$	(15,302	)	$	579,454	$	(15,603	)
December 31, 2016
Securities held-to-maturity:
Non-agency commercial mortgage-backed securities	$	60,221	$	(1,781	)	$	—	$	—		$	60,221	$	(1,781	)
Collateralized loan obligations	10,056		(6		)	56,095		(55		)	66,151		(61		)
Corporate debt securities	9,907		(91		)	—		—			9,907		(91		)
Total securities held-to-maturity	$	80,184	$	(1,878	)	$	56,095	$	(55	)	$	136,279	$	(1,933	)
Securities available-for-sale:
SBA loan pool securities	$	1,221	$	—		$	—	$	—		$	1,221	$	—
U.S. government agency and U.S. government sponsored enterprise residential mortgage-backed securities	805,803		(23,410		)	760		(8		)	806,563		(23,418		)
Non-agency residential mortgage-backed securities	116,216		(4,238		)	230		—			116,446		(4,238		)
Collateralized loan obligations	187,592		(674		)	—		—			187,592		(674		)
Corporate debt securities	—		—			3,530		(108		)	3,530		(108		)
Total securities available-for-sale	$	1,110,832	$	(28,322	)	$	4,520	$	(116	)	$	1,115,352	$	(28,438	)