3. Investment Securities

As of December 31, 2018 and 2017, investment securities consisted predominantly of the following investment categories:

U.S. Treasury and debt securities – includes U.S. Treasury notes and debt securities issued by government-sponsored enterprises.

Mortgage-backed securities – includes securities backed by notes or receivables secured by mortgage assets with cash flows based on actual or scheduled payments.

Collateralized mortgage obligations – includes securities backed by a pool of mortgages with cash flows distributed based on certain rules rather than pass through payments.

Debt securities issued by states and political subdivisions – includes general obligation bonds issued by state and local governments.

As of December 31, 2018 and 2017, all of the Company’s investment securities were classified as debt securities and available-for-sale. Amortized cost and fair value of securities as of December 31, 2018 and 2017 were as follows:


	2018								2017
	Amortized		Unrealized		Unrealized		Fair		Amortized		Unrealized		Unrealized		Fair
(dollars in thousands)	Cost		Gains		Losses		Value		Cost		Gains		Losses		Value
U.S. Treasury securities	$	389,470	$	—	$	—	$	389,470	$	404,376	$	—	$	(12,121)	$	392,255
Government-sponsored enterprises debt securities		248,372		—		(6,778)		241,594		249,712		—		(7,111)		242,601
Government agency mortgage-backed securities		426,710		—		(15,174)		411,536		356,858		—		(5,468)		351,390
Government-sponsored enterprises mortgage-backed securities		156,056		85		(5,294)		150,847		178,702		169		(4,130)		174,741
Collateralized mortgage obligations:
Government agency		2,779,620		—		(97,171)		2,682,449		3,367,173		47		(76,746)		3,290,474
Government-sponsored enterprises		620,337		—		(17,745)		602,592		779,911		25		(17,218)		762,718
Debt securities issued by states and political subdivisions		19,854		—		—		19,854		20,543		—		(64)		20,479
Total available-for-sale securities	$	4,640,419	$	85	$	(142,162)	$	4,498,342	$	5,357,275	$	241	$	(122,858)	$	5,234,658

Proceeds from both calls and sales of investment securities were nil for both the years ended December 31, 2018 and 2017. Proceeds from calls and sales of investment securities totaled $121.2 million and $825.4 million, respectively, for the year ended December 31, 2016. The Company recorded no gross realized gains and no gross realized losses for both the years ended December 31, 2018 and 2017, respectively. The Company recorded gross realized gains of $27.4 million and gross realized losses of $0.1 million for the year ended December 31, 2016. The income tax expense related to the Company’s net realized gains on the sale of investment securities was nil during both the years ended December 31, 2018 and 2017, respectively. The income tax expense related to the Company’s net realized gains on the sale of investment securities was $10.8 million for the year ended December 31, 2016. Gains and losses realized on sales of securities are determined using the specific identification method.

Interest income from taxable investment securities was $106.6 million, $102.3 million and $83.0 million for the years ended December 31, 2018, 2017 and 2016, respectively. Interest income from non-taxable investment securities was $0.5 million and $0.1 million for the years ended December 31, 2018 and 2017. The Company did not own any non-taxable investment securities during the year ended December 31, 2016.

The amortized cost and fair value of debt securities issued by the U.S. Treasury, government-sponsored enterprises and states and political subdivisions as of December 31, 2018, by contractual maturity, are shown below. Mortgage-backed securities and collateralized mortgage obligations are disclosed separately in the table below as remaining expected maturities will differ from contractual maturities as borrowers have the right to prepay obligations.


	December 31, 2018
	Amortized		Fair
(dollars in thousands)	Cost		Value
Due after one year through five years	$	523,117	$	521,453
Due after five years through ten years		114,725		109,611
Due after ten years		19,854		19,854
		657,696		650,918

Government agency mortgage-backed securities		426,710		411,536
Government-sponsored enterprises mortgage-backed securities		156,056		150,847
Collateralized mortgage obligations:
Government agency		2,779,620		2,682,449
Government-sponsored enterprises		620,337		602,592
Total mortgage-backed securities and collateralized mortgage obligations		3,982,723		3,847,424
Total available-for-sale securities	$	4,640,419	$	4,498,342

At December 31, 2018, pledged securities totaled $2.0 billion, of which $1.7 billion was pledged to secure public deposits and $232.7 million was pledged to secure other financial transactions. At December 31, 2017, pledged securities totaled $3.0 billion, of which $2.8 billion was pledged to secure public deposits and $229.2 million was pledged to secure other financial transactions.

The Company held no securities of any single issuer, other than debt securities issued by the U.S. government, government agency and government-sponsored enterprises, which were in excess of 10% of stockholders’ equity as of December 31, 2018 and 2017.

The following table presents the unrealized gross losses and fair values of securities in the available-for-sale portfolio by length of time that the 154 and 196 individual securities in each category have been in a continuous loss position as of December 31, 2018 and 2017, respectively. The unrealized losses on investment securities were attributable to market conditions.


	Time in Continuous Loss as of December 31, 2018
	Less Than 12 Months				12 Months or More				Total
	Unrealized				Unrealized				Unrealized
(dollars in thousands)	Losses		Fair Value		Losses		Fair Value		Losses		Fair Value
Government-sponsored enterprises debt securities	$	—	$	—	$	(6,778)	$	157,939	$	(6,778)	$	157,939
Government agency mortgage-backed securities		—		—		(15,174)		373,891		(15,174)		373,891
Government-sponsored enterprises mortgage-backed securities		(1)		172		(5,293)		125,869		(5,294)		126,041
Collateralized mortgage obligations:
Government agency		—		—		(97,171)		2,475,532		(97,171)		2,475,532
Government-sponsored enterprises		—		—		(17,745)		486,175		(17,745)		486,175
Total available-for-sale securities with unrealized losses	$	(1)	$	172	$	(142,161)	$	3,619,406	$	(142,162)	$	3,619,578


	Time in Continuous Loss as of December 31, 2017
	Less Than 12 Months				12 Months or More				Total
	Unrealized				Unrealized				Unrealized
(dollars in thousands)	Losses		Fair Value		Losses		Fair Value		Losses		Fair Value
U.S. Treasury securities	$	(994)	$	48,182	$	(11,127)	$	344,073	$	(12,121)	$	392,255
Government-sponsored enterprises debt securities		(642)		59,358		(6,469)		183,243		(7,111)		242,601
Government agency mortgage-backed securities		(976)		200,963		(4,492)		150,427		(5,468)		351,390
Government-sponsored enterprises mortgage-backed securities		(1)		63		(4,129)		168,342		(4,130)		168,405
Collateralized mortgage obligations:
Government agency		(23,236)		1,473,170		(53,510)		1,803,338		(76,746)		3,276,508
Government-sponsored enterprises		(3,203)		327,435		(14,015)		403,321		(17,218)		730,756
Debt securities issued by states and political subdivisions		(64)		10,641		—		—		(64)		10,641
Total available-for-sale securities with unrealized losses	$	(29,116)	$	2,119,812	$	(93,742)	$	3,052,744	$	(122,858)	$	5,172,556

Other-Than-Temporary Impairment

At December 31, 2018, the Company had the intent to sell 48 securities with an aggregate amortized cost basis of $898.2 million. As a result, the Company recorded an OTTI write-down of $24.1 million for the year ended December 31, 2018. The OTTI write-down represents the entire difference between the amortized cost basis and fair value of the securities as of December 31, 2018. In January 2019, the Company completed its sale of the 48 securities and recorded an additional loss of $2.6 million. As of December 31, 2017, the Company did not have any securities with the intent to sell and determined it was more likely than not that the Company would not be required to sell the securities prior to recovery of the amortized cost basis.

As the Company has the intent and ability to hold the remaining securities in an unrealized loss position as of December 31, 2018, each security with an unrealized loss position in the above tables has been further assessed to determine if a credit loss exists. If it is probable that the Company will not collect all amounts due according to the contractual terms of an investment security, an OTTI is considered to have occurred. In determining whether a credit loss exists, the Company estimates the present value of future cash flows expected to be collected from the investment security. If the present value of future cash flows is less than the amortized cost basis of the security, an OTTI exists. As of December 31, 2018 and 2017, the Company did not expect any credit losses in its debt securities and no OTTI related to credit losses were recognized on securities during the years ended December 31, 2018 and 2017.

The following table provides a detail of the OTTI write-downs included in earnings for the year ended December 31, 2018:


	Year Ended
(dollars in thousands)	December 31, 2018
U.S. Treasury securities	$	(13,634)
Government-sponsored enterprises debt securities		(1,344)
Government agency mortgage-backed securities		(146)
Government-sponsored enterprises mortgage-backed securities		(763)
Collateralized mortgage obligations:
Government agency		(5,064)
Government-sponsored enterprises		(2,711)
Debt securities issued by states and political subdivisions		(423)
Total OTTI write-downs included in earnings	$	(24,085)

Visa Class B Restricted Shares

In 2008, the Company received 394,000 Visa Class B restricted shares as part of Visa’s initial public offering. Visa Class B restricted shares are not currently convertible to publicly traded Visa Class A common shares, and only transferable in limited circumstances, until the settlement of certain litigation which are indemnified by Visa members, including the Company. As there are existing transfer restrictions and the outcome of the aforementioned litigation is uncertain, these shares were included in the consolidated balance sheets at their historical cost of $0.

In 2016, the Company recorded a $22.7 million net realized gain related to the sale of 274,000 Visa Class B restricted shares. Concurrent with the sale of the Visa Class B restricted shares, the Company entered into an agreement with the buyer that requires payment to the buyer in the event Visa reduces each member bank’s Class B conversion rate to unrestricted Class A common shares. On June 28, 2018, Visa additionally funded its litigation escrow account, thereby reducing each member bank’s Class B conversion rate to unrestricted Class A common shares. Accordingly, on July 5, 2018, Visa announced a decrease in conversion rate from 1.6483 to 1.6298 effective June 28, 2018. In July 2018, the Company made a payment of approximately $0.7 million to the buyer as a result of the reduction in the Visa Class B conversion rate. See “Note 17. Derivative Financial Instruments” in the notes to the consolidated financial statements included in Item 8. Financial Statements and Supplementary Data for more information.

The Company held approximately 120,000 Visa Class B restricted shares as of both December 31, 2018 and 2017. These shares continued to be carried at $0 cost basis during each of the respective periods.