INVESTMENTS

The amortized cost and fair value of investment securities classified as available for sale and held to maturity at December 31, 2017 and 2016, were as follows:



	December 31, 2017
(Dollars in thousands)	Cost		Gross unrealized gains		Gross unrealized losses		Fair value
Investment securities available for sale
U.S. Treasury	$	1,658,410	$	—	$	546	$	1,657,864
Mortgage-backed securities	5,428,074		1,544		80,192		5,349,426
Equity securities	75,471		29,737		—		105,208
Corporate bonds	59,414		557		8		59,963
Other	7,645		256		182		7,719
Total investment securities available for sale	$	7,229,014	$	32,094	$	80,928	$	7,180,180

	December 31, 2016
	Cost		Gross unrealized gains		Gross unrealized losses		Fair value
U.S. Treasury	$	1,650,675	$	579	$	935	$	1,650,319
Government agency	40,291		107		—		40,398
Mortgage-backed securities	5,259,466		2,809		86,850		5,175,425
Equity securities	71,873		11,634		—		83,507
Corporate bonds	49,367		195		—		49,562
Other	7,615		—		246		7,369
Total investment securities available for sale	$	7,079,287	$	15,324	$	88,031	$	7,006,580

	December 31, 2017
	Cost		Gross unrealized gains		Gross unrealized losses		Fair value
Investment securities held to maturity
Mortgage-backed securities	$	76	$	5	$	—	$	81

	December 31, 2016
	Cost		Gross unrealized gains		Gross unrealized losses		Fair value
Mortgage-backed securities	$	98	$	6	$	—	$	104

Investments in mortgage-backed securities primarily represent securities issued by the Government National Mortgage Association, Federal National Mortgage Association and Federal Home Loan Mortgage Corporation. Investments in equity securities and corporate bonds represent positions in securities of other financial institutions. Other investments include trust preferred securities of financial institutions. The following table provides the amortized cost and fair value by contractual maturity. Expected maturities will differ from contractual maturities on certain securities because borrowers and issuers may have the right to call or prepay obligations with or without prepayment penalties. Repayments of mortgage-backed securities are dependent on the repayments of the underlying loan balances. Equity securities do not have a stated maturity date.



	December 31, 2017				December 31, 2016
(Dollars in thousands)	Cost		Fair value		Cost		Fair value
Investment securities available for sale
Non-amortizing securities maturing in:
One year or less	$	808,768	$	808,301	$	842,798	$	842,947
One through five years	849,642		849,563		848,168		847,770
Five through 10 years	59,414		59,963		49,367		49,562
Over 10 years	7,645		7,719		7,615		7,369
Mortgage-backed securities	5,428,074		5,349,426		5,259,466		5,175,425
Equity securities	75,471		105,208		71,873		83,507
Total investment securities available for sale	$	7,229,014	$	7,180,180	$	7,079,287	$	7,006,580
Investment securities held to maturity
Mortgage-backed securities held to maturity	$	76	$	81	$	98	$	104

For each period presented, securities gains (losses) include the following:



	Year ended December 31
(Dollars in thousands)	2017			2016			2015
Gross gains on retirement/sales of investment securities available for sale	$	11,635		$	27,104		$	10,834
Gross losses on sales of investment securities available for sale	(7,342		)	(431		)	(17		)
Net securities gains	$	4,293		$	26,673		$	10,817

The following table provides information regarding securities with unrealized losses as of December 31, 2017 and 2016:



	December 31, 2017
	Less than 12 months				12 months or more				Total
(Dollars in thousands)	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
Investment securities available for sale:
U.S. Treasury	$	1,408,166	$	345	$	249,698	$	201	$	1,657,864	$	546
Mortgage-backed securities	2,334,102		20,923		2,725,933		59,269		5,060,035		80,192
Corporate bonds	5,025		8		—		—		5,025		8
Other	5,349		182		—		—		5,349		182
Total	$	3,752,642	$	21,458	$	2,975,631	$	59,470	$	6,728,273	$	80,928

	December 31, 2016
	Less than 12 months				12 months or more				Total
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
Investment securities available for sale:
U.S. Treasury	$	807,822	$	935	$	—	$	—	$	807,822	$	935
Mortgage-backed securities	4,442,700		82,161		362,351		4,689		4,805,051		86,850
Other	7,369		246		—		—		7,369		246
Total	$	5,257,891	$	83,342	$	362,351	$	4,689	$	5,620,242	$	88,031

Investment securities with an aggregate fair value of $2.98 billion have had continuous unrealized losses for more than 12 months as of December 31, 2017 with an aggregate unrealized loss of $59.5 million. As of December 31, 2017, 227 of these investments are government sponsored enterprise-issued mortgage-backed securities and 4 are U.S. Treasury securities.

None of the unrealized losses identified as of December 31, 2017 or December 31, 2016 relate to the marketability of the securities or the issuer’s ability to honor redemption obligations. Rather, the unrealized losses related to changes in interest rates relative to when the investment securities were purchased. For all periods presented, BancShares had the ability and intent to retain these securities for a period of time sufficient to recover all unrealized losses. Therefore, none of the securities were deemed to be other than temporarily impaired.

Investment securities having an aggregate carrying value of $4.59 billion at December 31, 2017 and $4.55 billion at December 31, 2016 were pledged as collateral to secure public funds on deposit and certain short-term borrowings, and for other purposes as required by law.