Securities (Tables)

6 Months Ended

Jun. 30, 2017

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, 2017								December 31, 2016
	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	$	—	$	—	$	—	$	—	$	249,889	$	1,762	$	—	$	251,651
Residential mortgage-backed securities	3,992		32		24		4,000		4,511		39		—		4,550
States and political subdivisions	1,590,920		37,288		2,891		1,625,317		1,994,710		16,821		6,335		2,005,196
Other	1,350		—		2		1,348		1,350		—		—		1,350
Total	$	1,596,262	$	37,320	$	2,917	$	1,630,665	$	2,250,460	$	18,622	$	6,335	$	2,262,747
Available for Sale
U.S. Treasury	$	4,203,901	$	27,541	$	7,188	$	4,224,254	$	4,003,692	$	24,984	$	8,945	$	4,019,731
Residential mortgage-backed securities	681,364		25,693		1,436		705,621		756,072		30,388		1,293		785,167
States and political subdivisions	5,734,600		122,028		45,286		5,811,342		5,403,918		50,101		98,134		5,355,885
Other	42,520		—		—		42,520		42,494		—		—		42,494
Total	$	10,662,385	$	175,262	$	53,910	$	10,783,737	$	10,206,176	$	105,473	$	108,372	$	10,203,277

Securities, with Unrealized Losses Segregated by Length of Impairment

As of June 30, 2017, 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	$	2,191	$	24	$	—	$	—	$	2,191	$	24
States and political subdivisions	20,127		83		118,937		2,808		139,064		2,891
Other	1,348		2		—		—		1,348		2
Total	$	23,666	$	109	$	118,937	$	2,808	$	142,603	$	2,917
Available for Sale
U.S. Treasury	$	1,587,812	$	7,188	$	—	$	—	$	1,587,812	$	7,188
Residential mortgage-backed securities	67,596		1,204		5,866		232		73,462		1,436
States and political subdivisions	1,450,982		38,138		142,507		7,148		1,593,489		45,286
Total	$	3,106,390	$	46,530	$	148,373	$	7,380	$	3,254,763	$	53,910

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, 2017 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	$	329,237	$	334,156	$	57,171	$	57,528
Due after one year through five years	170,170		177,977		4,818,303		4,845,635
Due after five years through ten years	349,324		357,485		362,030		375,143
Due after ten years	743,539		757,047		4,700,997		4,757,290
Residential mortgage-backed securities	3,992		4,000		681,364		705,621
Equity securities	—		—		42,520		42,520
Total	$	1,596,262	$	1,630,665	$	10,662,385	$	10,783,737

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,
	2017		2016		2017		2016
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,
	2017			2016		2017			2016
Proceeds from sales	$	8,247,439		$	—	$	8,247,439		$	1,060,196
Gross realized gains	—			—		—			11,133
Gross realized losses	(50		)	—		(50		)	—
Tax (expense) benefit of securities gains/losses	18			—		18			(3,897		)

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,
	2017			2016			2017			2016
Premium amortization	$	(24,119	)	$	(22,219	)	$	(48,147	)	$	(44,559	)
Discount accretion	2,105			3,138			4,495			5,753
Net (premium amortization) discount accretion	$	(22,014	)	$	(19,081	)	$	(43,652	)	$	(38,806	)

Trading Account Securities, at Estimated Fair Value

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

	June 30, 2017		December 31, 2016
U.S. Treasury	$	17,726	$	16,594
States and political subdivisions	9,874		109
Total	$	27,600	$	16,703

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,
	2017			2016			2017			2016
Net gain on sales transactions	$	293		$	351		$	604		$	653
Net mark-to-market gains (losses)	(56		)	(2		)	(43		)	(1		)
Net gain (loss) on trading account securities	$	237		$	349		$	561		$	652