Investment Securities (Tables)

9 Months Ended

Sep. 30, 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:

	September 30, 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	$	557,060	$	1,321	$	(4,319	)	$	554,062
Obligations of states and political subdivisions		121,635		1,182		(1,600	)		121,217
Marketable equity securities		3,000		—		(43	)		2,957
Total securities available for sale	$	681,695	$	2,503	$	(5,962	)	$	678,236
Securities Held to Maturity
Obligations of U.S. government corporations and agencies	$	521,999	$	6,837	$	(1,442	)	$	527,394
Obligations of states and political subdivisions		14,568		211		(50	)		14,729
Total securities held to maturity	$	536,567	$	7,048	$	(1,492	)	$	542,123
	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. 

Investment Securities	Available for Sale				Held to Maturity
(In thousands)	Amortized		Estimated		Amortized		Estimated
	Cost		Fair Value		Cost		Fair Value
Due in one year	$	2	$	2		—		—
Due after one year through five years		206		206	$	1,199	$	1,233
Due after five years through ten years		2,223		2,279		4,098		4,165
Due after ten years		679,264		675,749		531,270		536,725
Totals	$	681,695	$	678,236	$	536,567	$	542,123

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
	Fair		Unrealized			Fair		Unrealized			Fair		Unrealized
September 30, 2017	Value		Loss			Value		Loss			Value		Loss
						(in thousands)
Securities Available for Sale:
Obligations of U.S. government corporations and agencies	$	365,252	$	(4,319	)		—		—		$	365,252	$	(4,319	)
Obligations of states and political subdivisions		31,937		(903	)	$	8,905	$	(697	)		40,842		(1,600	)
Marketable equity securities		2,957		(43	)		—		—			2,957		(43	)
Total securities available-for-sale	$	400,146	$	(5,265	)	$	8,905	$	(697	)	$	409,051	$	(5,962	)
Securities Held to Maturity:
Obligations of U.S. government corporations and agencies	$	133,453	$	(1,442	)		—		—		$	133,453	$	(1,442	)
Obligations of states and political subdivisions		3,197		(50	)		—		—			3,179		(50	)
Total securities held-to-maturity	$	136,650	$	(1,492	)		—		—		$	136,650	$	(1,492	)
	Less than 12 months					12 months or more					Total
	Fair		Unrealized			Fair		Unrealized			Fair		Unrealized
December 31, 2016	Value		Loss			Value		Loss			Value		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	)