Investment Securities (Tables)

3 Months Ended

Mar. 31, 2018

Investments Schedule [Abstract]

Amortized Cost and Estimated Fair Values of Investments in Debt Securities

The amortized cost and estimated fair values of investments in debt securities are summarized in the following tables:

	March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Debt Securities Available for Sale
Obligations of U.S. government corporations and agencies	$	633,310	$	317	$	(16,970	)	$	616,657
Obligations of states and political subdivisions		121,560		466		(2,788	)		119,238
Total debt securities available for sale	$	754,870	$	783	$	(19,758	)	$	735,895
Debt Securities Held to Maturity
Obligations of U.S. government corporations and agencies	$	481,457	$	533	$	(7,737	)	$	474,253
Obligations of states and political subdivisions		14,578		39		(231	)		14,386
Total debt securities held to maturity	$	496,035	$	572	$	(7,968	)	$	488,639
	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Debt Securities Available for Sale
Obligations of U.S. government corporations and agencies	$	609,695	$	695	$	(5,601	)	$	604,789
Obligations of states and political subdivisions		121,597		1,888		(329	)		123,156
Total debt securities available for sale	$	731,292	$	2,583	$	(5,930	)	$	727,945
Debt Securities Held to Maturity
Obligations of U.S. government corporations and agencies	$	500,271	$	5,101	$	(1,889	)	$	503,483
Obligations of states and political subdivisions		14,573		146		(37	)		14,682
Total debt securities held to maturity	$	514,844	$	5,247	$	(1,926	)	$	518,165

Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity

For purposes of the following table, the entire outstanding balance of these mortgage-backed securities issued by U.S. government corporations and agencies is categorized based on final maturity date.  

Debt Securities	Available for Sale				Held to Maturity
(In thousands)	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Due in one year	$	2	$	2	$	—	$	—
Due after one year through five years		235		236		1,215		1,228
Due after five years through ten years		3,324		3,452		20,631		20,291
Due after ten years		751,309		732,205		474,189		467,120
Totals	$	754,870	$	735,895	$	496,035	$	488,639

Gross Unrealized Losses on Debt Securities

Gross unrealized losses on debt 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 were as follows:

	Less than 12 months					12 months or more					Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
						(in thousands)
March 31, 2018
Debt Securities Available for Sale
Obligations of U.S. government corporations and agencies	$	434,302	$	(10,345	)	$	158,324	$	(6,625	)	$	592,626	$	(16,970	)
Obligations of states and political subdivisions		65,047		(1,456	)		16,042		(1,332	)		81,089		(2,788	)
Total debt securities available for sale	$	499,349	$	(11,801	)	$	174,366	$	(7,957	)	$	673,715	$	(19,758	)
Debt Securities Held to Maturity
Obligations of U.S. government corporations and agencies	$	299,575	$	(4,551	)	$	89,809	$	(3,186	)	$	389,384	$	(7,737	)
Obligations of states and political subdivisions		8,901		(144	)		2,579		(87	)		11,480		(231	)
Total debt securities held to maturity	$	308,476	$	(4,695	)	$	92,388	$	(3,273	)	$	400,864	$	(7,968	)
	Less than 12 months					12 months or more					Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
						(in thousands)
December 31, 2017
Debt Securities Available for Sale
Obligations of U.S. government corporations and agencies	$	284,367	$	(2,176	)	$	166,338	$	(3,425	)	$	450,705	$	(5,601	)
Obligations of states and political subdivisions		4,904		(35	)		17,085		(294	)		21,989		(329	)
Total securities available for sale	$	289,271	$	(2,211	)	$	183,423	$	(3,719	)	$	472,694	$	(5,930	)
Debt Securities Held to Maturity
Obligations of U.S. government corporations and agencies	$	93,017	$	(567	)	$	95,367	$	(1,322	)	$	188,384	$	(1,889	)
Obligations of states and political subdivisions		1,488		(7	)		2,637		(30	)		4,125		(37	)
Total debt securities held to maturity	$	94,505	$	(574	)	$	98,004	$	(1,352	)	$	192,509	$	(1,926	)