Investment Securities (Tables)

9 Months Ended

Sep. 30, 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:

September 30, 2017	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
U.S. Treasury notes	$	3,206	$	1	$	(2	)	$	3,205
Obligations of U.S. government agencies	508,449		338		(3,209		)	505,578
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,431,125		8,829		(4,716		)	1,435,238
Private mortgage-backed securities	97,715		313		(190		)	97,838
Corporate securities	66,754		126		(2		)	66,878
Other investments	2,951		9		—			2,960
Total	$	2,110,200	$	9,616	$	(8,119	)	$	2,111,697

September 30, 2017	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	180,981	$	3,726	$	(123	)	$	184,584
U.S agency residential mortgage-backed securities & collateralized mortgage obligations	243,380		10,725		(10,172		)	243,933
Obligations of U.S. government agencies	19,737		43		(19		)	19,761
Corporate securities	61,704		272		(33		)	61,943
Other investments	183		1		—			184
Total	$	505,985	$	14,767	$	(10,347	)	$	510,405

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
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
Corporate securities	53,032		139		(211		)	52,960
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 September 30,						Nine Months Ended September 30,
	2017			2016			2017			2016
Gross realized gains	$	1,072		$	239		$	1,079		$	404
Gross realized losses	(301		)	(14		)	(301		)	(92		)

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 September 30, 2017 and December 31, 2016:

	Less than 12 Months					12 Months or More					Total
September 30, 2017	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
U.S. Treasury notes	$	2,201	$	(2	)	$	—	$	—		$	2,201	$	(2	)
Obligations of U.S. government agencies	303,060		(2,458		)	86,718		(751		)	389,778		(3,209		)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	556,771		(3,731		)	74,732		(985		)	631,503		(4,716		)
Private mortgage-backed securities	30,518		(189		)	31		(1		)	30,549		(190		)
Corporate securities	6,987		(2		)	—		—			6,987		(2		)
Total	$	899,537	$	(6,382	)	$	161,481	$	(1,737	)	$	1,061,018	$	(8,119	)

	Less than 12 Months					12 Months or More					Total
September 30, 2017	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	25,207	$	(97	)	$	4,208	$	(26	)	$	29,415	$	(123	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	38,712		(8,814		)	18,846		(1,358		)	57,558		(10,172		)
Obligations of U.S. government agencies	9,972		(19		)	—		—			9,972		(19		)
Corporate securities	3,966		—			5,017		(33		)	8,983		(33		)
Total	$	77,857	$	(8,930	)	$	28,071	$	(1,417	)	$	105,928	$	(10,347	)

	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 September 30, 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
September 30, 2017	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	375,199	$	376,264	$	87,077	$	83,899
After one year but within five years	1,287,854		1,288,384		264,417		272,442
After five years but within ten years	278,782		278,277		119,579		121,039
After ten years	168,365		168,772		34,912		33,025
Total	$	2,110,200	$	2,111,697	$	505,985	$	510,405