Debt Securities Available for Sale and Debt Securities Held to Maturity

Debt securities were as follows:



	At December 31,
	2018								2017
(In thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Debt securities available for sale:
Mortgage-backed securities:
U.S. Government sponsored enterprises and federal agencies	$	1,930,696	$	9,222	$	26,728	$	1,913,190	$	908,189	$	308	$	13,812	$	894,685
Other	4		—		—		4		6		—		—		6
Obligations of states and political subdivisions	566,304		46		9,479		556,871		810,159		7,967		3,799		814,327
Total debt securities available for sale	$	2,497,004	$	9,268	$	36,207	$	2,470,065	$	1,718,354	$	8,275	$	17,611	$	1,709,018
Debt securities held to maturity:
Mortgage-backed securities:
U.S. Government sponsored enterprises and federal agencies	$	146,052	$	1,460	$	1,045	$	146,467	$	158,776	$	4,462	$	412	$	162,826
Other securities	2,800		—		—		2,800		2,800		—		—		2,800
Total debt securities held to maturity	$	148,852	$	1,460	$	1,045	$	149,267	$	161,576	$	4,462	$	412	$	165,626

At December 31, 2018 and 2017, mortgage-backed debt securities with a carrying value of $1.6 million and $0.9 million, respectively, were pledged as collateral to secure certain deposits and borrowings.

We have assessed each debt security with unrealized losses included in the table above for credit impairment. As part of that assessment we evaluated and concluded that it is more likely than not that we will not be required to and do not intend to sell any of the debt securities prior to recovery of the amortized cost. Unrealized losses on debt securities available for sale and debt securities held to maturity were due to changes in interest rates.

Net gains (losses) on debt securities were $348 thousand, $237 thousand and $(581) thousand for 2018, 2017 and 2016, respectively. During 2018, TCF sold $251.3 million of debt securities available for sale. There were no sales of debt securities available for sale in 2017 and 2016. There were no impairment charges recognized on debt securities available for sale in 2018, 2017 and 2016 and no impairment charges recognized on debt securities held to maturity in 2018 and 2017. The net gains on debt securities in 2018 and 2017 were primarily related to recoveries on previously impaired debt securities held to maturity and included net gains of $127 thousand on the sale of debt securities available for sale in 2018. The net loss on debt securities in 2016 was primarily due to impairment charges of $716 thousand recognized on debt securities held to maturity.

Gross unrealized losses and fair value of debt securities available for sale and debt securities held to maturity aggregated by investment category and the length of time the securities were in a continuous loss position were as follows:



	At December 31, 2018
	Less than 12 months				12 months or more				Total
(In thousands)	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
Debt securities available for sale:
Mortgage-backed securities:
U.S. Government sponsored enterprises and federal agencies	$	102,709	$	184	$	838,482	$	26,544	$	941,191	$	26,728
Obligations of states and political subdivisions	3,620		—		526,817		9,479		530,437		9,479
Total debt securities available for sale	$	106,329	$	184	$	1,365,299	$	36,023	$	1,471,628	$	36,207

Debt securities held to maturity:
Mortgage-backed securities:
U.S. Government sponsored enterprises and federal agencies	$	3,074	$	14	$	31,738	$	1,031	$	34,812	$	1,045
Total debt securities held to maturity	$	3,074	$	14	$	31,738	$	1,031	$	34,812	$	1,045



	At December 31, 2017
	Less than 12 months				12 months or more				Total
(In thousands)	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
Debt securities available for sale:
Mortgage-backed securities:
U.S. Government sponsored enterprises and federal agencies	$	406,298	$	2,686	$	428,585	$	11,126	$	834,883	$	13,812
Obligations of states and political subdivisions	103,759		486		207,516		3,313		311,275		3,799
Total debt securities available for sale	$	510,057	$	3,172	$	636,101	$	14,439	$	1,146,158	$	17,611

Debt securities held to maturity:
Mortgage-backed securities:
U.S. Government sponsored enterprises and federal agencies	$	13,309	$	132	$	11,470	$	280	$	24,779	$	412
Total debt securities held to maturity	$	13,309	$	132	$	11,470	$	280	$	24,779	$	412

The amortized cost and fair value of debt securities available for sale and debt securities held to maturity by final contractual maturity were as follows. The final contractual maturities do not consider possible prepayments and therefore expected maturities may differ because borrowers may have the right to prepay.



	At December 31,
	2018				2017
(In thousands)	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Debt securities available for sale:
Due in one year or less	$	—	$	—	$	6	$	6
Due in 1-5 years	24,464		24,375		15,178		15,312
Due in 5-10 years	509,832		503,768		514,336		517,867
Due after 10 years	1,962,708		1,941,922		1,188,834		1,175,833
Total debt securities available for sale	$	2,497,004	$	2,470,065	$	1,718,354	$	1,709,018

Debt securities held to maturity:
Due in one year or less	$	—	$	—	$	1,000	$	1,000
Due in 1-5 years	2,400		2,400		1,400		1,400
Due in 5-10 years	430		432		400		400
Due after 10 years	146,022		146,435		158,776		162,826
Total debt securities held to maturity	$	148,852	$	149,267	$	161,576	$	165,626

Interest income attributable to debt securities available for sale was as follows:



	Year Ended December 31,
(In thousands)	2018		2017		2016
Taxable interest income	$	37,436	$	18,382	$	16,238
Tax-exempt interest income	17,138		14,896		10,335
Total interest income	$	54,574	$	33,278	$	26,573