Securities (Tables)

6 Months Ended

Jun. 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.

	June 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	$	249,661	$	5,461	$	—	$	255,122	$	249,441	$	7,776	$	—	$	257,217
Residential mortgage-backed securities	5,156		63		—		5,219		6,456		63		4		6,515
States and political subdivisions	2,083,435		72,954		3,697		2,152,692		2,405,762		46,003		6,149		2,445,616
Other	1,350		—		—		1,350		1,350		—		13		1,337
Total	$	2,339,602	$	78,478	$	3,697	$	2,414,383	$	2,663,009	$	53,842	$	6,166	$	2,710,685
Available for Sale
U.S. Treasury	$	3,984,495	$	135,522	$	—	$	4,120,017	$	3,980,986	$	22,041	$	8,507	$	3,994,520
Residential mortgage-backed securities	861,794		46,504		113		908,185		1,000,024		42,142		734		1,041,432
States and political subdivisions	4,695,526		277,835		7		4,973,354		3,996,113		133,305		1,459		4,127,959
Other	42,477		—		—		42,477		42,447		—		—		42,447
Total	$	9,584,292	$	459,861	$	120	$	10,044,033	$	9,019,570	$	197,488	$	10,700	$	9,206,358

Securities, with Unrealized Losses Segregated by Length of Impairment

As of June 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
Residential mortgage-backed securities	$	—	$	—	$	—	$	—	$	—	$	—
States and political subdivisions	37,489		512		154,689		3,185		192,178		3,697
Total	$	37,489	$	512	$	154,689	$	3,185	$	192,178	$	3,697
Available for Sale
Residential mortgage-backed securities	$	139	$	3	$	16,844	$	110	$	16,983	$	113
States and political subdivisions	10,340		7		—		—		10,340		7
Total	$	10,479	$	10	$	16,844	$	110	$	27,323	$	120

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 June 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	$	608,032	$	621,611	$	59,059	$	59,905
Due after one year through five years	349,978		371,006		3,778,355		3,873,477
Due after five years through ten years	277,818		287,257		1,204,980		1,282,404
Due after ten years	1,098,618		1,129,290		3,637,627		3,877,585
Residential mortgage-backed securities	5,156		5,219		861,794		908,185
Equity securities	—		—		42,477		42,477
Total	$	2,339,602	$	2,414,383	$	9,584,292	$	10,044,033

Sales of Securities Held-to-Maturity

Sales of securities held to maturity were as follows:

	Three Months Ended   June 30,				Six Months Ended   June 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   June 30,				Six Months Ended   June 30,
	2016		2015		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   June 30,						Six Months Ended   June 30,
	2016			2015			2016			2015
Premium amortization	$	(22,219	)	$	(20,832	)	$	(44,559	)	$	(40,838	)
Discount accretion	3,138			2,629			5,753			5,327
Net (premium amortization) discount accretion	$	(19,081	)	$	(18,203	)	$	(38,806	)	$	(35,511	)

Trading Account Securities, at Estimated Fair Value

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

	June 30, 2016		December 31, 2015
U.S. Treasury	$	13,381	$	16,443
States and political subdivisions	14,553		136
Total	$	27,934	$	16,579

Net Gains and Losses on Trading Account Securities

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

	Three Months Ended   June 30,						Six Months Ended   June 30,
	2016			2015			2016			2015
Net gain on sales transactions	$	351		$	278		$	653		$	558
Net mark-to-market gains (losses)	(2		)	(32		)	(1		)	(46		)
Net gain (loss) on trading account securities	$	349		$	246		$	652		$	512