Investment Securities (Tables)

3 Months Ended

Mar. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Amortized Cost and Approximate Fair Values of Investment Securities

The amortized cost and approximate fair values of investment securities are summarized as follows:

March 31, 2017	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
U.S. Treasury notes	$	3,608	$	5	$	(1	)	$	3,612
Obligations of U.S. government agencies	411,096		449		(4,302		)	407,243
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,243,751		8,257		(8,345		)	1,243,663
Private mortgage-backed securities	106		1		(1		)	106
Other investments	2,951		16		—			2,967
Total	$	1,661,512	$	8,728	$	(12,649	)	$	1,657,591

March 31, 2017	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	153,679	$	3,121	$	(222	)	$	156,578
Corporate securities	52,918		248		(67		)	53,099
Obligations of U.S. government agencies	19,737		—		(82		)	19,655
U.S agency residential mortgage-backed securities &     collateralized mortgage obligations	264,427		9,044		(9,086		)	264,385
Other investments	207		1		—			208
Total	$	490,968	$	12,414	$	(9,457	)	$	493,925

December 31, 2016	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
U.S. Treasury notes	$	3,608	$	5	$	(1	)	$	3,612
Obligations of U.S. government agencies	397,411		343		(6,457		)	391,297
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,220,890		6,412		(13,601		)	1,213,701
Private mortgage-backed securities	116		1		(1		)	116
Other investments	2,951		21		—			2,972
Total	$	1,624,976	$	6,782	$	(20,060	)	$	1,611,698

December 31, 2016	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	160,192	$	2,723	$	(542	)	$	162,373
Corporate securities	53,032		139		(211		)	52,960
Obligations of U.S. government agencies	19,737		—		(162		)	19,575
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	279,578		7,804		(9,249		)	278,133
Other investments	231		1		—			232
Total	$	512,770	$	10,667	$	(10,164	)	$	513,273

Realized Gains (Losses) on Investments

Gross realized gains and losses from the disposition of investment securities are summarized in the following table:

	Three Months Ended March 31,
	2017		2016
Gross realized gains	$	2	$	57
Gross realized losses	—		(78		)

Gross Unrealized Losses and Fair Values of Investment Securities

The following tables show the gross unrealized losses and fair values of investment securities, aggregated by investment category, and the length of time individual investment securities have been in a continuous unrealized loss position, as of March 31, 2017 and December 31, 2016:

	Less than 12 Months					12 Months or More					Total
March 31, 2017	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
U.S. Treasury notes	$	598	$	(1	)	$	—	$	—		$	598	$	(1	)
Obligations of U.S. government agencies	324,693		(4,287		)	9,984		(15		)	334,677		(4,302		)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	652,089		(7,765		)	23,282		(580		)	675,371		(8,345		)
Private mortgage-backed securities	—		—			43		(1		)	43		(1		)
Total	$	977,380	$	(12,053	)	$	33,309	$	(596	)	$	1,010,689	$	(12,649	)

	Less than 12 Months					12 Months or More					Total
March 31, 2017	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	23,353	$	(218	)	$	2,442	$	(4	)	$	25,795	$	(222	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	77,626		(7,816		)	18,509		(1,270		)	96,135		(9,086		)
Obligations of U.S. government agencies	19,655		(82		)	—		—			19,655		(82		)
Corporate securities	16,061		(67		)	—		—			16,061		(67		)
Total	$	136,695	$	(8,183	)	$	20,951	$	(1,274	)	$	157,646	$	(9,457	)

	Less than 12 Months					12 Months or More					Total
December 31, 2016	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
U.S. Treasury notes	$	598	$	(1	)	$	—	$	—		$	598	$	(1	)
Obligations of U.S. government agencies	316,511		(6,457		)	—		—			316,511		(6,457		)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	746,265		(13,102		)	15,801		(499		)	762,066		(13,601		)
Private mortgage-backed securities	—		—			48		(1		)	48		(1		)
Total	$	1,063,374	$	(19,560	)	$	15,849	$	(500	)	$	1,079,223	$	(20,060	)

	Less than 12 Months					12 Months or More					Total
December 31, 2016	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	42,518	$	(533	)	$	2,831	$	(9	)	$	45,349	$	(542	)
Obligations of U.S. government agencies	19,575		(162		)	—		—			19,575		(162		)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	108,857		(7,973		)	19,986		(1,276		)	128,843		(9,249		)
Corporate securities	32,474		(211		)	—		—			32,474		(211		)
Total	$	203,424	$	(8,879	)	$	22,817	$	(1,285	)	$	226,241	$	(10,164	)

Maturities of Investment Securities

Maturities of investment securities at March 31, 2017 are shown below. Maturities of mortgage-backed securities have been adjusted to reflect shorter maturities based upon estimated prepayments of principal. All other investment securities maturities are shown at contractual maturity dates.

	Available-for-Sale				Held-to-Maturity
March 31, 2017	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	326,690	$	326,645	$	75,598	$	76,786
After one year but within five years	1,108,186		1,105,388		278,753		278,372
After five years but within ten years	164,443		163,370		109,051		110,987
After ten years	62,193		62,188		27,566		27,780
Total	$	1,661,512	$	1,657,591	$	490,968	$	493,925