Investment Securities (Tables)

9 Months Ended

Sep. 30, 2014

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, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
Obligations of U.S. government agencies	$	733,144	$	1,193	$	(5,840	)	$	728,497
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	851,119		11,152		(4,609		)	857,662
Private mortgage-backed securities	340		5		(1		)	344
Total	$	1,584,603	$	12,350	$	(10,450	)	$	1,586,503

September 30, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	189,919	$	6,260	$	(454	)	$	195,725
Corporate securities	21,860		82		—			21,942
U.S agency residential mortgage-backed securities &     collateralized mortgage obligations	370,933		1,081		(2,208		)	369,806
Other investments	559		—		—			559
Total	$	583,271	$	7,423	$	(2,662	)	$	588,032

December 31, 2013	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
Obligations of U.S. government agencies	$	774,055	$	1,432	$	(12,249	)	$	763,238
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,197,295		11,905		(25,147		)	1,184,053
Private mortgage-backed securities	407		9		(1		)	415
Total	$	1,971,757	$	13,346	$	(37,397	)	$	1,947,706

December 31, 2013	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	185,818	$	4,043	$	(2,049	)	$	187,812
Corporate securities	18,019		103		(8		)	18,114
Total	$	203,837	$	4,146	$	(2,057	)	$	205,926

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,
	2014			2013		2014			2013
Gross realized gains	$	—		$	30	$	243		$	42
Gross realized losses	(8		)	—		(163		)	(16		)

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, 2014 and December 31, 2013.

	Less than 12 Months					12 Months or More					Total
September 30, 2014	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
Obligations of U.S. government agencies	$	29,552	$	(209	)	$	388,327	$	(5,631	)	$	417,879	$	(5,840	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	102,237		(428		)	197,313		(4,181		)	299,550		(4,609		)
Private mortgage-backed securities	—		—			91		(1		)	91		(1		)
Total	$	131,789	$	(637	)	$	585,731	$	(9,813	)	$	717,520	$	(10,450	)

	Less than 12 Months					12 Months or More					Total
September 30, 2014	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	—	$	—		$	20,586	$	(454	)	$	20,586	$	(454	)
Private mortgage-backed securities	240,262		(2,208		)	—		—			240,262		(2,208		)
Total	$	240,262	$	(2,208	)	$	20,586	$	(454	)	$	260,848	$	(2,662	)

	Less than 12 Months					12 Months or More					Total
December 31, 2013	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
Obligations of U.S. government agencies	$	458,385	$	(10,355	)	$	59,362	$	(1,894	)	$	517,747	$	(12,249	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	634,199		(17,273		)	166,930		(7,874		)	801,129		(25,147		)
Private mortgage-backed securities	—		—			104		(1		)	104		(1		)
Total	$	1,092,584	$	(27,628	)	$	226,396	$	(9,769	)	$	1,318,980	$	(37,397	)

	Less than 12 Months					12 Months or More					Total
December 31, 2013	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	37,550	$	(1,319	)	$	14,296	$	(730	)	$	51,846	$	(2,049	)
Corporate securities	7,294		(8		)	—		—			7,294		(8		)
Total	$	44,844	$	(1,327	)	$	14,296	$	(730	)	$	59,140	$	(2,057	)

Maturities of Investment Securities

Maturities of investment securities at September 30, 2014 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, 2014	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	261,829	$	263,597	$	88,305	$	88,490
After one year but within five years	1,074,666		1,075,712		285,622		286,860
After five years but within ten years	181,258		179,828		144,409		146,353
After ten years	66,850		67,366		64,935		66,329
Total	$	1,584,603	$	1,586,503	$	583,271	$	588,032