Debt Securities Available for Sale and Debt Securities Held to Maturity (Tables)

12 Months Ended

Dec. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Schedule of Debt Securities

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

Schedule of Gross Unrealized Losses and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity that are in a Loss Position

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

Schedule of Amortized Cost and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity by Contractual Maturity

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

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