Investment Securities (Tables)

6 Months Ended

Jun. 30, 2013

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:

June 30, 2013	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
Obligations of U.S. government agencies	$	744,215	$	1,158	$	(11,559	)	$	733,814
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,190,975		15,783		(17,819		)	1,188,939
Private mortgage-backed securities	457		9		(3		)	463
Total	$	1,935,647	$	16,950	$	(29,381	)	$	1,923,216

June 30, 2013	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to Maturity:
State, county and municipal securities	$	197,193	$	5,730	$	(1,990	)	$	200,933
Corporate securities	18,130		64		(99		)	18,095
Total	$	215,323	$	5,794	$	(2,089	)	$	219,028

December 31, 2012	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
Obligations of U.S. government agencies	$	751,501	$	3,518	$	(163	)	$	754,856
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,214,377		27,000		(1,526		)	1,239,851
Private mortgage-backed securities	539		13		(1		)	551
Total	$	1,966,417	$	30,531	$	(1,690	)	$	1,995,258

December 31, 2012	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to Maturity:
State, county and municipal securities	$	192,875	$	10,835	$	(176	)	$	203,534
Corporate securities	14,975		64		(13		)	15,026
Other securities	373		—		—			373
Total	$	208,223	$	10,899	$	(189	)	$	218,933

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 June 30,				Six Months Ended June 30,
	2013		2012		2013		2012
Gross realized gains	$	4	$	201	$	12	$	232
Gross realized losses	16		3		16		3

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 June 30, 2013 and December 31, 2012

	Less than 12 Months					12 Months or More					Total
June 30, 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	$	442,307	$	(11,559	)	$	—	$	—		$	442,307	$	(11,559	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	742,630		(17,819		)	—		—			742,630		(17,819		)
Private mortgage-backed securities	—		—			112		(3		)	112		(3		)
Total	$	1,184,937	$	(29,378	)	$	112	$	(3	)	$	1,185,049	$	(29,381	)

	Less than 12 Months					12 Months or More					Total
June 30, 2013	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	52,225	$	(1,963	)	$	504	$	(27	)	$	52,729	$	(1,990	)
Corporate securities	12,399		(99		)	—		—			12,399		(99		)
Total	$	64,624	$	(2,062	)	$	504	$	(27	)	$	65,128	$	(2,089	)

	Less than 12 Months					12 Months or More					Total
December 31, 2012	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
Obligations of U.S. government agencies	$	93,982	$	(163	)	$	—	$	—		$	93,982	$	(163	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	250,198		(1,526		)	—		—			250,198		(1,526		)
Private mortgage-backed securities	—		—			137		(1		)	137		(1		)
Total	$	344,180	$	(1,689	)	$	137	$	(1	)	$	344,317	$	(1,690	)

	Less than 12 Months					12 Months or More					Total
December 31, 2012	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	19,389	$	(168	)	$	557	$	(8	)	$	19,946	$	(176	)
Corporate securities	9,312		(13		)	—		—			9,312		(13		)
Total	$	28,701	$	(181	)	$	557	$	(8	)	$	29,258	$	(189	)

Maturities of Investment Securities

Maturities of investment securities at June 30, 2013 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
June 30, 2013	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	328,967	$	328,602	$	4,898	$	4,935
After one year but within five years	944,490		940,392		57,519		58,058
After five years but within ten years	602,366		594,500		88,943		90,890
After ten years	59,824		59,722		63,963		65,145
Total	$	1,935,647	$	1,923,216	$	215,323	$	219,028