(2) Investment Securities

The amortized cost and estimated fair value by type of investment security at December 31, 2017 are as follows:


	Held to Maturity
			Gross		Gross
	Amortized		unrealized		unrealized		Estimated		Carrying
	cost		gains		losses		fair value		value
	(Dollars in Thousands)
Other securities	$	2,400	$	—	$	—	$	2,400	$	2,400
Total investment securities	$	2,400	$	—	$	—	$	2,400	$	2,400


	Available for Sale
			Gross		Gross
	Amortized		unrealized		unrealized		Estimated		Carrying
	cost		gains		losses		fair value		value(1)
	(Dollars in Thousands)
Residential mortgage-backed securities	$	3,943,092	$	14,110	$	(65,969)	$	3,891,233	$	3,891,233
Obligations of states and political subdivisions		225,096		7,871		(16)		232,951		232,951
Equity securities		28,075		293		(482)		27,886		27,886
Total investment securities	$	4,196,263	$	22,274	$	(66,467)	$	4,152,070	$	4,152,070

(1)

Included in the carrying value of residential mortgage‑ backed securities are $654,063 of mortgage‑backed securities issued by Ginnie Mae and $3,237,170 of mortgage‑backed securities issued by Fannie Mae and Freddie Mac

The amortized cost and estimated fair value of investment securities at December 31, 2017 , by contractual maturity, are shown below. Expected maturities will differ from contractual maturities because borrowers may have the right to prepay obligations with or without prepayment penalties.


	Held to Maturity				Available for Sale
	Amortized		Estimated		Amortized		Estimated
	Cost		fair value		Cost		fair value
	(Dollars in Thousands)
Due in one year or less	$	2,275	$	2,275	$	—	$	—
Due after one year through five years		125		125		—		—
Due after five years through ten years		—		—		1,939		1,967
Due after ten years		—		—		223,157		230,984
Residential mortgage-backed securities		—		—		3,943,092		3,891,233
Equity securities		—		—		28,075		27,886
Total investment securities	$	2,400	$	2,400	$	4,196,263	$	4,152,070

The amortized cost and estimated fair value by type of investment security at December 31, 2016 are as follows:


	Held to Maturity
			Gross		Gross
	Amortized		unrealized		unrealized		Estimated		Carrying
	cost		gains		losses		fair value		value
	(Dollars in Thousands)
Other securities	$	2,400	$	—	$	—	$	2,400	$	2,400
Total investment securities	$	2,400	$	—	$	—	$	2,400	$	2,400


	Available for Sale
			Gross		Gross		Estimated
	Amortized		unrealized		unrealized		fair		Carrying
	cost		gains		losses		value		value(1)
	(Dollars in Thousands)
Residential mortgage-backed securities	$	3,946,144	$	18,246	$	(69,920)	$	3,894,470	$	3,894,470
Obligations of states and political subdivisions		244,622		10,783		(433)		254,972		254,972
Equity securities		28,075		314		(482)		27,907		27,907
Total investment securities	$	4,218,841	$	29,343	$	(70,835)	$	4,177,349	$	4,177,349

(1)

Included in the carrying value of residential mortgage‑ backed securities are $850,033 of mortgage‑backed securities issued by Ginnie Mae, $3,026,832 of mortgage‑backed securities issued by Fannie Mae and Freddie Mac and $17,605 issued by non‑government entities

Residential mortgage‑backed securities are securities issued by Freddie Mac, Fannie Mae, Ginnie Mae or non‑government entities. Investments in residential mortgage‑backed securities issued by Ginnie Mae are fully guaranteed by the U.S. government. Investments in mortgage‑backed securities issued by Freddie Mac and Fannie Mae are not fully guaranteed by the U.S. government; however, the Company believes that the quality of the bonds is similar to other AAA rated bonds with limited credit risk, particularly given the placement of Fannie Mae and Freddie Mac into conservatorship by the federal government in early September 2008 and because securities issued by others that are collateralized by residential mortgage‑backed securities issued by Fannie Mae and Freddie Mac are rated consistently as AAA rated securities.

The amortized cost and fair value of available for sale investment securities pledged to qualify for fiduciary powers, to secure public monies as required by law, repurchase agreements and short‑term fixed borrowings was $1,428,387,000 and $1,403,084,000, respectively, at December 31, 2017.

Proceeds from the sale and call of securities available‑for‑sale were $396,066,000, $352,743,000 and $164,163,000 during 2017, 2016 and 2015, respectively, which amounts included $377,756,000, $338,138,000 and $128,444,000 of mortgage‑backed securities. Gross gains of $1,186,000, $586,000 and $2,450,000, and gross losses of $5,960,000, $3,212,000 and $6,132,000 were realized on the sales in 2017, 2016 and 2015, respectively.

Gross unrealized losses on investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, at December 31, 2017 were as follows:


	Less than 12 months				12 months or more				Total
			Unrealized				Unrealized				Unrealized
	Fair Value		Losses		Fair Value		Losses		Fair Value		Losses
	(Dollars in Thousands)
Available for sale:
Residential mortgage-backed securities	$	1,061,577	$	(13,157)	$	2,029,455	$	(52,812)	$	3,091,032	$	(65,969)
Obligations of states and political subdivisions		5,534		(9)		522		(7)		6,056		(16)
Equity securities		11,499		(251)		8,019		(231)		19,518		(482)
	$	1,078,610	$	(13,417)	$	2,037,996	$	(53,050)	$	3,116,606	$	(66,467)

Gross unrealized losses on investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual securities have been in a continuous loss position, at December 31, 2016 were as follows:


	Less than 12 months				12 months or more				Total
			Unrealized				Unrealized				Unrealized
	Fair Value		Losses		Fair Value		Losses		Fair Value		Losses
	(Dollars in Thousands)
Available for sale:
Residential mortgage-backed securities	$	2,513,872	$	(52,245)	$	396,695	$	(17,675)	$	2,910,567	$	(69,920)
Obligations of states and political subdivisions		31,104		(433)		—		—		31,104		(433)
Equity securities		14,066		(184)		5,452		(298)		19,518		(482)
	$	2,559,042	$	(52,862)	$	402,147	$	(17,973)	$	2,961,189	$	(70,835)

The unrealized losses on investments in residential mortgage‑backed securities are primarily caused by changes in market interest rates. Residential mortgage‑backed securities are primarily securities issued by Freddie Mac, Fannie Mae and Ginnie Mae. The contractual cash obligations of the securities issued by Ginnie Mae are fully guaranteed by the U.S. government. The contractual cash obligations of the securities issued by Freddie Mac and Fannie Mae are not fully guaranteed by the U.S. government; however, the Company believes that the quality of the bonds is similar to other AAA rated bonds with limited credit risk, particularly given the placement of Fannie Mae and Freddie Mac into conservatorship by the federal government in early September 2008 and because securities issued by others that are collateralized by residential mortgage‑backed securities issued by Fannie Mae and Freddie Mac are rated consistently as AAA rated securities. The decrease in fair value on residential mortgage‑backed securities issued by Freddie Mac, Fannie Mae and Ginnie Mae is due to market interest rates. The Company has no intent to sell and more likely than not be required to sell before a market price recovery or maturity of the securities; therefore, it is the conclusion of the Company that the investments in residential mortgage‑backed securities issued by Freddie Mac, Fannie Mae and Ginnie Mae are not considered other‑than‑temporarily impaired. In addition, the Company had a small investment in non‑agency residential mortgage‑backed securities that had additional market volatility beyond economically induced interest rate events, which were sold in the first quarter of 2017. The Company concluded that the investments in non‑agency residential mortgage‑backed securities were other‑than‑temporarily impaired due to both credit and other than credit issues. No impairment charges were recorded in 2017. Impairment charges of $354,000 ($230,100, after tax) and $954,000 ($620,1000, after tax) were recorded in 2016 and 2015, respectively on the non‑agency residential mortgage backed securities. The impairment charges represent the credit related impairment on the securities.

The unrealized losses on investments in other securities are caused by fluctuations in market interest rates. The underlying cash obligations of the securities are guaranteed by the entity underwriting the debt instrument. It is the belief of the Company that the entity issuing the debt will honor its interest payment schedule, as well as the full debt at maturity. The securities are purchased by the Company for their economic value. The decrease in fair value is primarily due to market interest rates and not other factors, and because the Company has no intent to sell and will more likely than not be required to sell before a market price recovery or maturity of the securities, it is the conclusion of the Company that the investments are not considered other‑than‑temporarily impaired.

The following table presents a reconciliation of credit‑related impairment charges on available‑for‑sale investments recognized in earnings for the twelve months ended December 31, 2017 (in Thousands):


Balance at December 31, 2016	$	13,931
Sale of other-than-temporarily impaired available-for-sale securities during period		(13,931)
Balance at December 31, 2017	$	—


Balance at December 31, 2015	$	13,577
Impairment charges recognized during period		354
Balance at December 31, 2016	$	13,931

The following table presents a reconciliation of credit-related impairment charges on available-for-sale investments recognized in earnings for the twelve months ended December 31, 2015 (in Thousands):


Balance at December 31, 2014	$	12,623
Impairment charges recognized during period		954
Balance at December 31, 2015	$	13,577