The amortized cost, gross unrealized gains and losses and estimated fair values for available-for-sale and held-to-maturity securities by major security type at December 31, 2018 and December 31, 2017 were as follows (in thousands):



December 31, 2018	Amortized Cost		Gross Unrealized Gains		Gross Unrealized (Losses)			Fair Value
Available-for-sale:
U.S. Treasury securities and obligations of U.S. government corporations & agencies	$	201,380	$	504	$	(3,235	)	$	198,649
Obligations of states and political subdivisions	193,195		1,224		(1,840		)	192,579
Mortgage-backed securities: GSE residential	304,372		486		(6,186		)	298,672
Other securities	2,278		96		—			2,374
Total available-for-sale	$	701,225	$	2,310	$	(11,261	)	$	692,274
Held-to-maturity:
U.S. Treasury securities and obligations of U.S. government corporations & agencies	$	69,436	$	—	$	(1,527	)	$	67,909
December 31, 2017
Available-for-sale:
U.S. Treasury securities and obligations of U.S. government corporations & agencies	$	115,796	$	8	$	(2,034	)	$	113,770
Obligations of states and political subdivisions	165,037		2,254		(1,025		)	166,266
Mortgage-backed securities: GSE residential	295,778		493		(2,460		)	293,811
Trust preferred securities	2,893		—		(345		)	2,548
Other securities	2,039		145		—			2,184
Total available-for-sale	$	581,543	$	2,900	$	(5,864	)	$	578,579
Held-to-maturity:
U.S. Treasury securities and obligations of U.S. government corporations & agencies	$	69,332	$	103	$	(978	)	$	68,457

Trust preferred securities at December 31, 2017, is a trust preferred pooled security issued by First Tennessee Financial (“FTN”). The unrealized loss of this security, which had a maturity of twenty years, was primarily due to its long-term nature, a lack of demand or inactive market for the security, and concerns regarding the underlying financial institutions that have issued the trust preferred security. This security was sold during 2018. See the heading “Trust Preferred Securities” below for further information regarding this security.

Proceeds from sales of investment securities, realized gains and losses and income tax expense were as follows during the years ended December 31, 2018, 2017 and 2016 (in thousands):



	2018			2017			2016
Proceeds from sales	$	13,152		$	159,663		$	70,757
Gross gains	941			773			1,192
Gross losses	(40		)	(157		)	—
Income tax expense	261			216			465

The following table indicates the expected maturities of investment securities classified as available-for-sale presented at fair value, and held-to-maturity presented at amortized cost at December 31, 2018 and the weighted average yield for each range of maturities (in thousands):



	One year or less			After 1 through 5 years			After 5 through 10 years			After ten years			Total
Available-for-sale:
U.S. Treasury securities and obligations of U.S. government corporations and agencies	$	148,656		$	49,993		$	—		$	—		$	198,649
Obligations of state and political subdivisions	23,282			89,930			78,294			1,073			192,579
Mortgage-backed securities: GSE residential	622			160,900			137,150			—			298,672
Other securities	—			2,010			—			364			2,374
Total investments	$	172,560		$	302,833		$	215,444		$	1,437		$	692,274
Weighted average yield	2.57		%	2.83		%	2.93		%	3.10		%	2.80		%
Full tax-equivalent yield	2.70		%	3.14		%	3.34		%	4.14		%	3.10		%
Held-to-maturity:
U.S. Treasury securities and obligations of U.S. government corporations and agencies	$	39,995		$	29,441		$	—		$	—		$	69,436
Weighted average yield	1.76		%	2.08		%	—		%	—		%	1.90		%
Full tax-equivalent yield	1.76		%	2.08		%	—		%	—		%	1.90		%

The weighted average yields are calculated on the basis of the amortized cost and effective yields weighted for the scheduled maturity of each security. Tax-equivalent yields have been calculated using a 21% tax rate. With the exception of obligations of the U.S. Treasury and other U.S. government agencies and corporations, there were no investment securities of any single issuer, the book value of which exceeded 10% of stockholders' equity at December 31, 2018.

Investment securities carried at approximately $628 million and $479 million at December 31, 2018 and 2017, respectively, were pledged to secure public deposits and repurchase agreements and for other purposes as permitted or required by law.

The following table presents the aging of gross unrealized losses and fair value by investment category as of December 31, 2018 and 2017 (in thousands):



	Less than 12 months					12 months or more					Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
December 31, 2018
Available-for-sale:
U.S. Treasury securities and obligations of U.S. government corporations and agencies	$	16,095	$	(148	)	$	105,549	$	(3,087	)	$	121,644	$	(3,235	)
Obligations of states and political subdivisions	38,782		(450		)	42,741		(1,390		)	81,523		(1,840		)
Mortgage-backed securities: GSE residential	81,435		(1,150		)	171,321		(5,036		)	252,756		(6,186		)
Total	$	136,312	$	(1,748	)	$	319,611	$	(9,513	)	$	455,923	$	(11,261	)
Held-to-maturity:
U.S. Treasury securities and obligations of U.S. government corporations and agencies	$	19,683	$	(147	)	$	48,226	$	(1,380	)	$	67,909	$	(1,527	)
December 31, 2017
U.S. Treasury securities and obligations of U.S. government corporations and agencies	$	58,584	$	(540	)	$	47,972	$	(1,494	)	$	106,556	$	(2,034	)
Obligations of states and political subdivisions	42,618		(769		)	9,267		(256		)	51,885		(1,025		)
Mortgage-backed securities: GSE residential	187,949		(1,942		)	22,609		(518		)	210,558		(2,460		)
Trust preferred securities	—		—			2,548		(345		)	2,548		(345		)
Total	$	289,151	$	(3,251	)	$	82,396	$	(2,613	)	$	371,547	$	(5,864	)
Held-to-maturity:
U.S. Treasury securities and obligations of U.S. government corporations and agencies	$	34,101	$	(525	)	$	14,540	$	(453	)	$	48,641	$	(978	)

U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies. At December 31, 2018, there were twenty-three available-for-sale U.S. Treasury securities and obligations of U.S. government corporations and agencies with a fair value of $105,549,000 and unrealized losses of $3,087,000 in a continuous unrealized loss position for twelve months or more. At December 31, 2017, there were eleven available-for-sale U.S. Treasury securities and obligations of U.S. government corporations and agencies with a fair value of $47,972,000 and unrealized losses of $1,494,000 in a continuous unrealized loss position for twelve months or more. At December 31, 2018 there were nine held-to-maturity U.S. Treasury securities and obligations of U.S. government corporations and agencies with a fair value of $48,226,000 and unrealized losses of $1,380,000 in a continuous unrealized loss position for twelve months or more. At December 31, 2017 there were seven held-to maturity U.S. Treasury securities and obligations of U.S. government corporations and agencies with a fair value of $14,540,000 and unrealized losses of $453,000 in a continuous unrealized loss position for twelve months or more.

Obligations of states and political subdivisions. At December 31, 2018 there were eighty-four obligations of states and political subdivisions with a fair value of $42,741,000 and unrealized losses of $1,390,000 in a continuous unrealized loss position for twelve months or more. At December 31, 2017, there were thirty-nine obligations of states and political subdivisions with a fair value of $9,267,000 and unrealized losses of $256,000 in a continuous unrealized loss position for twelve months or more.

Mortgage-backed Securities: GSE Residential. At December 31, 2018 there were sixty-nine mortgage-backed securities with a fair value of $171,321,000 and unrealized losses of $5,036,000 in a continuous unrealized loss position for twelve months or more. At December 31, 2017, there were twenty-six mortgage-backed security with a fair value of $22,609,000 and unrealized losses of $518,000 in a continuous unrealized loss position for twelve months or more.

Trust Preferred Securities. At December 31, 2017, there was one trust preferred securities with a fair value of $2,548,000 and unrealized losses of $345,000 in a continuous unrealized loss position for twelve months or more. These unrealized losses were primarily due to the long-term nature of the trust preferred securities, a lack of demand or inactive market for these securities, the impending change to the regulatory treatment of these securities, and concerns regarding the underlying financial institutions that have issued the trust preferred securities. The Company sold this security during 2018.

Other securities. At December 31, 2018 or 2017 there were no other securities in a continuous unrealized loss position for twelve months or more.

The Company does not believe any other individual unrealized loss as of December 31, 2018 represents OTTI. However, given the continued disruption in the financial markets, the Company may be required to recognize OTTI losses in future periods with respect to its available for sale investment securities portfolio. The amount and timing of any additional OTTI will depend on the decline in the underlying cash flows of the securities. Should the impairment of any of these securities become other-than-temporary, the cost basis of the investment will be reduced and the resulting loss recognized in the period the other-than-temporary impairment is identified.

Other-than-temporary Impairment

Upon acquisition of a security, the Company determines whether it is within the scope of the accounting guidance for investments in debt and equity securities or whether it must be evaluated for impairment under the accounting guidance for beneficial interests in securitized financial assets.

Credit Losses Recognized on Investments

As described above, the Company’s investments in trust preferred securities experienced fair value deterioration due to credit losses but were not otherwise other-than-temporarily impaired. The following table provides information about those trust preferred securities for which only a credit loss was recognized in income and other losses were recorded in other comprehensive income (loss) for the years ended December 31, 2018, 2017 and 2016 (in thousands).



	Accumulated Credit Losses as of December 31:
	2018			2017		2016
Credit losses on trust preferred securities held:
Beginning of period	$	1,111		$	1,111	$	1,111
Additions related to OTTI losses not previously recognized	—			—		—
Reductions due to sales / (recoveries)	(1,111		)	—		—
Reductions due to change in intent or likelihood of sale	—			—		—
Additions related to increases in previously recognized OTTI losses	—			—		—
Reductions due to increases in expected cash flows	—			—		—
End of period	$	—		$	1,111	$	1,111

Maturities of investment securities were as follows at December 31, 2018 (in thousands):



	Amortized Cost		Estimated Fair Value
Available-for-sale:
Due in one year or less	$	174,049	$	171,938
Due after one-five years	142,357		141,933
Due after five-ten years	79,095		78,294
Due after ten years	1,352		1,437
	396,853		393,602
Mortgage-backed securities: GSE residential	304,372		298,672
Total available-for-sale	701,225		692,274
Held-to-maturity:
Due in one year or less	39,995		38,790
Due after one-five years	29,441		29,119
Due after five-ten years	—		—
Due after ten years	—		—
Total held-to-maturity	$	69,436	$	67,909

Expected maturities will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.