Investment Securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments Schedule [Abstract]

Amortized Cost and Estimated Fair Values of Investments in Debt and Equity Securities

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

	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
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
Marketable equity securities		3,000		—		(62	)		2,938
Total securities available for sale	$	734,292	$	2,583	$	(5,992	)	$	730,883
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 securities held to maturity	$	514,844	$	5,247	$	(1,926	)	$	518,165
	December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Securities Available for Sale
Obligations of U.S. government corporations and agencies	$	434,357	$	1,949	$	(6,628	)	$	429,678
Obligations of states and political subdivisions		121,746		267		(4,396	)		117,617
Marketable equity securities		3,000		—		(62	)		2,938
Total securities available for sale	$	559,103	$	2,216	$	(11,086	)	$	550,233
Securities Held to Maturity
Obligations of U.S. government corporations and agencies	$	587,982	$	5,001	$	(4,199	)	$	588,784
Obligations of states and political subdivisions		14,554		56		(191	)		14,419
Total securities held to maturity	$	602,536	$	5,057	$	(4,390	)	$	603,203

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.  

	Available for Sale				Held to Maturity
Investment Securities (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		211		211	$	1,207	$	1,226
Due after five years through ten years		2,231		2,281		14,011		14,067
Due after ten years		731,848		728,389		499,626		502,872
Totals	$	734,292	$	730,883	$	514,844	$	518,165

Gross Unrealized Losses on Investment

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, were as follows:

	Less than 12 months					12 months or more					Total
December 31, 2017:	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
	(in thousands)
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	)
Marketable equity securities		—		—			2,938		(62	)		2,938		(62	)
Total securities available-for-sale	$	289,271	$	(2,211	)	$	186,361	$	(3,781	)	$	475,632	$	(5,992	)
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 securities held-to-maturity	$	94,505	$	(574	)	$	98,004	$	(1,352	)	$	192,509	$	(1,926	)

	Less than 12 months					12 months or more				Total
December 31, 2016:	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
	(in thousands)
Securities Available for Sale
Obligations of U.S. government corporations and agencies	$	370,389	$	(6,628	)		—		—	$	370,389	$	(6,628	)
Obligations of states and political subdivisions		90,825		(4,396	)		—		—		90,825		(4,396	)
Marketable equity securities		2,938		(62	)		—		—		2,938		(62	)
Total securities available-for-sale	$	464,152	$	(11,086	)		—		—	$	464,152	$	(11,086	)
Securities Held to Maturity
Obligations of U.S. government corporations and agencies	$	280,497	$	(4,199	)		—		—	$	280,497	$	(4,199	)
Obligations of states and political subdivisions		9,984		(191	)		—		—		9,984		(191	)
Total securities held-to-maturity	$	290,481	$	(4,390	)		—		—	$	290,481	$	(4,390	)