Securities (Tables)

3 Months Ended

Mar. 31, 2016

Investments, Debt and Equity Securities [Abstract]

Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities

A summary of the amortized cost and estimated fair value of securities, excluding trading securities, is presented below.

	March 31, 2016								December 31, 2015
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Held to Maturity
U.S. Treasury	$	249,550	$	6,895	$	—	$	256,445	$	249,441	$	7,776	$	—	$	257,217
Residential mortgage-backed securities	5,756		71		2		5,825		6,456		63		4		6,515
States and political subdivisions	2,111,363		55,139		5,088		2,161,414		2,405,762		46,003		6,149		2,445,616
Other	1,350		—		—		1,350		1,350		—		13		1,337
Total	$	2,368,019	$	62,105	$	5,090	$	2,425,034	$	2,663,009	$	53,842	$	6,166	$	2,710,685
Available for Sale
U.S. Treasury	$	3,731,803	$	93,031	$	—	$	3,824,834	$	3,980,986	$	22,041	$	8,507	$	3,994,520
Residential mortgage-backed securities	930,927		45,929		216		976,640		1,000,024		42,142		734		1,041,432
States and political subdivisions	4,067,577		156,585		876		4,223,286		3,996,113		133,305		1,459		4,127,959
Other	42,469		—		—		42,469		42,447		—		—		42,447
Total	$	8,772,776	$	295,545	$	1,092	$	9,067,229	$	9,019,570	$	197,488	$	10,700	$	9,206,358

Securities, with Unrealized Losses Segregated by Length of Impairment

As of March 31, 2016, securities with unrealized losses, segregated by length of impairment, were as follows:

	Less than 12 Months				More than 12 Months				Total
	Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses
Held to Maturity
Residential mortgage-backed securities	$	2,352	$	2	$	—	$	—	$	2,352	$	2
States and political subdivisions	60,055		1,016		142,187		4,072		202,242		5,088
Total	$	62,407	$	1,018	$	142,187	$	4,072	$	204,594	$	5,090
Available for Sale
Residential mortgage-backed securities	$	227	$	2	$	24,321	$	214	$	24,548	$	216
States and political subdivisions	164,280		449		67,609		427		231,889		876
Total	$	164,507	$	451	$	91,930	$	641	$	256,437	$	1,092

Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity

The amortized cost and estimated fair value of securities, excluding trading securities, at March 31, 2016 are presented below by contractual maturity. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Residential mortgage-backed securities and equity securities are shown separately since they are not due at a single maturity date.

	Held to Maturity				Available for Sale
	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Due in one year or less	$	576,622	$	594,737	$	55,687	$	57,014
Due after one year through five years	323,002		343,717		3,504,555		3,572,118
Due after five years through ten years	291,767		297,659		1,235,012		1,293,154
Due after ten years	1,170,872		1,183,096		3,004,126		3,125,834
Residential mortgage-backed securities	5,756		5,825		930,927		976,640
Equity securities	—		—		42,469		42,469
Total	$	2,368,019	$	2,425,034	$	8,772,776	$	9,067,229

Sales of Securities Held-to-Maturity

Sales of securities held to maturity were as follows:

	Three Months Ended   March 31,
	2016			2015
Proceeds from sales	$	135,610		$	—
Amortized cost	131,840			—
Gross realized gains	3,770			—
Gross realized losses	—			—
Tax (expense) benefit of securities gains/losses	(1,319		)	—

Sales of Securities Available for Sale

Sales of securities available for sale were as follows:

	Three Months Ended   March 31,
	2016			2015
Proceeds from sales	$	1,060,196		$	223,987
Gross realized gains	11,133			228
Gross realized losses	—			—
Tax (expense) benefit of securities gains/losses	(3,897		)	(80		)

Premium Amortization and Discount Accretion Included in Income on Securities [Table Text Block]

Premium amortization and discount accretion included in interest income on securities was as follows:

	Three Months Ended   March 31,
	2016			2015
Premium amortization	$	(22,340	)	$	(20,006	)
Discount accretion	2,615			2,698
Net (premium amortization) discount accretion	$	(19,725	)	$	(17,308	)

Trading Account Securities, at Estimated Fair Value

Trading account securities, at estimated fair value, were as follows:

	March 31, 2016		December 31, 2015
U.S. Treasury	$	16,544	$	16,443
States and political subdivisions	479		136
Total	$	17,023	$	16,579

Net Gains and Losses on Trading Account Securities

Net gains and losses on trading account securities were as follows:

	Three Months Ended   March 31,
	2016		2015
Net gain on sales transactions	$	302	$	280
Net mark-to-market gains (losses)	1		(14		)
Net gain (loss) on trading account securities	$	303	$	266