Securities (Tables)

9 Months Ended

Sep. 30, 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.

	September 30, 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	$	249,774	$	3,683	$	—	$	253,457	$	249,441	$	7,776	$	—	$	257,217
Residential mortgage-backed securities	4,633		68		—		4,701		6,456		63		4		6,515
States and political subdivisions	2,008,703		62,028		3,809		2,066,922		2,405,762		46,003		6,149		2,445,616
Other	1,350		—		—		1,350		1,350		—		13		1,337
Total	$	2,264,460	$	65,779	$	3,809	$	2,326,430	$	2,663,009	$	53,842	$	6,166	$	2,710,685
Available for Sale
U.S. Treasury	$	3,984,791	$	114,958	$	—	$	4,099,749	$	3,980,986	$	22,041	$	8,507	$	3,994,520
Residential mortgage-backed securities	805,227		44,388		72		849,543		1,000,024		42,142		734		1,041,432
States and political subdivisions	4,966,187		210,657		5,794		5,171,050		3,996,113		133,305		1,459		4,127,959
Other	42,484		—		—		42,484		42,447		—		—		42,447
Total	$	9,798,689	$	370,003	$	5,866	$	10,162,826	$	9,019,570	$	197,488	$	10,700	$	9,206,358

Securities, with Unrealized Losses Segregated by Length of Impairment

As of September 30, 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
States and political subdivisions	$	3,084	$	1	$	186,220	$	3,808	$	189,304	$	3,809
Total	$	3,084	$	1	$	186,220	$	3,808	$	189,304	$	3,809
Available for Sale
Residential mortgage-backed securities	$	8,539	$	28	$	7,050	$	44	$	15,589	$	72
States and political subdivisions	565,166		5,794		—		—		565,166		5,794
Total	$	573,705	$	5,822	$	7,050	$	44	$	580,755	$	5,866

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 September 30, 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	$	687,531	$	700,574	$	82,116	$	83,878
Due after one year through five years	279,353		296,816		4,109,251		4,204,075
Due after five years through ten years	323,435		333,322		848,652		899,310
Due after ten years	969,508		991,017		3,910,959		4,083,536
Residential mortgage-backed securities	4,633		4,701		805,227		849,543
Equity securities	—		—		42,484		42,484
Total	$	2,264,460	$	2,326,430	$	9,798,689	$	10,162,826

Sales of Securities Held-to-Maturity

Sales of securities held to maturity were as follows:

	Three Months Ended   September 30,				Nine Months Ended   September 30,
	2016		2015		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   September 30,						Nine Months Ended   September 30,
	2016			2015			2016			2015
Proceeds from sales	$	7,980,049		$	4,497,202		$	9,040,245		$	4,721,189
Gross realized gains	1			—			11,134			228
Gross realized losses	(38		)	(52		)	(38		)	(52		)
Tax (expense) benefit of securities gains/losses	13			18			(3,884		)	(62		)

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   September 30,						Nine Months Ended   September 30,
	2016			2015			2016			2015
Premium amortization	$	(22,762	)	$	(21,354	)	$	(67,321	)	$	(62,192	)
Discount accretion	2,497			2,519			8,250			7,846
Net (premium amortization) discount accretion	$	(20,265	)	$	(18,835	)	$	(59,071	)	$	(54,346	)

Trading Account Securities, at Estimated Fair Value

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

	September 30, 2016		December 31, 2015
U.S. Treasury	$	16,161	$	16,443
States and political subdivisions	9,538		136
Total	$	25,699	$	16,579

Net Gains and Losses on Trading Account Securities

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

	Three Months Ended   September 30,				Nine Months Ended   September 30,
	2016		2015		2016			2015
Net gain on sales transactions	$	379	$	277	$	1,032		$	835
Net mark-to-market gains (losses)	—		6		(1		)	(40		)
Net gain (loss) on trading account securities	$	379	$	283	$	1,031		$	795