Investment Securities (Tables)

12 Months Ended

Dec. 31, 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:

December 31, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale
Obligations of U.S. government agencies	$	725,408	$	895	$	(5,370	)	$	720,933
U.S. agency residential mortgage-backed securities &    collateralized mortgage obligations	982,764		11,526		(3,624		)	990,666
Private mortgage-backed securities	322		5		(2		)	325
Total	$	1,708,494	$	12,426	$	(8,996	)	$	1,711,924

December 31, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to Maturity
State, county and municipal securities	$	188,941	$	5,949	$	(386	)	$	194,504
Corporate securities	32,565		54		(75		)	32,544
U.S agency residential mortgage-backed securities & collateralized mortgage obligations	353,176		5,563		(1,758		)	356,981
Other investments	504		—		—			504
Total	$	575,186	$	11,566	$	(2,219	)	$	584,533

Gross gains of $274 and gross losses of $213 were realized on the disposition of available-for-sale securities in 2014.

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

Gross Unrealized Losses and Fair Values of Investment Securities

The following table shows 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 December 31, 2014 and 2013.

	Less than 12 Months					12 Months or More					Total
December 31, 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	$	135,888	$	(702	)	$	309,283	$	(4,668	)	$	445,171	$	(5,370	)
U.S. agency residential mortgage-backed    securities & collateralized mortgage    obligations	219,214		(887		)	151,380		(2,737		)	370,594		(3,624		)
Private mortgage-backed securities	—		—			90		(2		)	90		(2		)
Total	$	355,102	$	(1,589	)	$	460,753	$	(7,407	)	$	815,855	$	(8,996	)

	Less than 12 Months					12 Months or More					Total
December 31, 2014	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity
State, county and municipal securities	$	7,979	$	(13	)	$	20,097	$	(373	)	$	28,076	$	(386	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	61,201		(1,758		)	—		—			61,201		(1,758		)
Corporate securities	14,755		(75		)	—		—			14,755		(75		)
Total	$	83,935	$	(1,846	)	$	20,097	$	(373	)	$	104,032	$	(2,219	)

	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

All other investment securities maturities are shown at contractual maturity dates.

	Available-for-Sale				Held-to-Maturity
December 31, 2014	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	252,677	$	254,560	$	85,751	$	86,983
After one year but within five years	1,218,143		1,219,799		283,000		286,783
After five years but within ten years	188,520		188,015		150,601		153,287
After ten years	49,154		49,550		55,834		57,480
Total	$	1,708,494	$	1,711,924	$	575,186	$	584,533