Securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Year End Securities Held To Maturity And Available For Sale

Year-end securities held to maturity and available for sale consisted of the following:

	2017								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	3,610		15		38		3,587		4,511		39		—		4,550
States and political subdivisions	1,428,488		26,462		2,746		1,452,204		1,994,710		16,821		6,335		2,005,196
Other	—		—		—		—		1,350		—		—		1,350
Total	$	1,432,098	$	26,477	$	2,784	$	1,455,791	$	2,250,460	$	18,622	$	6,335	$	2,262,747
Available for Sale:
U.S. Treasury	$	3,453,391	$	7,494	$	15,732	$	3,445,153	$	4,003,692	$	24,984	$	8,945	$	4,019,731
Residential mortgage-backed securities	648,288		19,048		2,250		665,086		756,072		30,388		1,293		785,167
States and political subdivisions	6,185,711		167,293		16,795		6,336,209		5,403,918		50,101		98,134		5,355,885
Other	42,561		—		—		42,561		42,494		—		—		42,494
Total	$	10,329,951	$	193,835	$	34,777	$	10,489,009	$	10,206,176	$	105,473	$	108,372	$	10,203,277

Year End Securities with Unrealized Losses, Segregated by Length of Impairment

Year-end 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
2017
Held to Maturity:
Residential mortgage-backed securities	$	2,694	$	38	$	—	$	—	$	2,694	$	38
States and political subdivisions	28,591		58		74,113		2,688		102,704		2,746
Total	$	31,285	$	96	$	74,113	$	2,688	$	105,398	$	2,784
Available for Sale:
U.S. Treasury	$	2,336,081	$	9,861	$	517,575	$	5,871	$	2,853,656	$	15,732
Residential mortgage-backed securities	144,264		949		45,436		1,301		189,700		2,250
States and political subdivisions	148,575		1,194		838,329		15,601		986,904		16,795
Total	$	2,628,920	$	12,004	$	1,401,340	$	22,773	$	4,030,260	$	34,777

	Less than 12 Months				More than 12 Months				Total
	Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses
2016
Held to Maturity:
States and political subdivisions	$	425,896	$	2,596	$	183,245	$	3,739	$	609,141	$	6,335
Total	$	425,896	$	2,596	$	183,245	$	3,739	$	609,141	$	6,335
Available for Sale:
U.S. Treasury	$	1,421,216	$	8,945	$	—	$	—	$	1,421,216	$	8,945
Residential mortgage-backed securities	81,442		1,031		6,413		262		87,855		1,293
States and political subdivisions	2,695,997		98,134		—		—		2,695,997		98,134
Total	$	4,198,655	$	108,110	$	6,413	$	262	$	4,205,068	$	108,372

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 December 31, 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	$	264,587	$	267,924	$	57,390	$	58,057
Due after one year through five years	159,765		163,819		4,063,797		4,056,508
Due after five years through ten years	386,711		391,784		425,148		439,008
Due after ten years	617,425		628,677		5,092,767		5,227,789
Residential mortgage-backed securities	3,610		3,587		648,288		665,086
Equity securities	—		—		42,561		42,561
Total	$	1,432,098	$	1,455,791	$	10,329,951	$	10,489,009

Held-to-maturity Securities [Table Text Block]

Sales of securities held to maturity were as follows:

	2017		2016			2015
Proceeds from sales	$	—	$	136,719		$	—
Amortized cost	—		132,974			—
Gross realized gains	—		3,770			—
Gross realized losses	—		(25		)	—
Tax expense related to securities gains/losses	—		(1,311		)	—

Sales of Securities Available for Sale

Sales of securities available for sale were as follows:

	2017			2016			2015
Proceeds from sales	$	11,963,359		$	14,847,380		$	12,683,169
Gross realized gains	1			13,289			228
Gross realized losses	(4,942		)	(2,059		)	(159		)
Tax benefit (expense) related to securities gains/losses	1,729			(3,931		)	(24		)

Premium Amortization and Discount Accretion Included in Income on Securities

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

	2017			2016			2015
Premium amortization	$	(97,841	)	$	(90,782	)	$	(84,467	)
Discount accretion	7,908			11,077			10,682
Net (premium amortization) discount accretion	$	(89,933	)	$	(79,705	)	$	(73,785	)

Year End Trading Account Securities, at Estimated Fair Value

Year-end trading account securities, at estimated fair value, were as follows:

	2017		2016
U.S. Treasury	$	19,210	$	16,594
States and political subdivisions	1,888		109
Total	$	21,098	$	16,703

Net Gains and Losses on Trading Account Securities

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

	2017			2016			2015
Net gain on sales transactions	$	1,408		$	1,236		$	1,109
Net mark-to-market gains (losses)	(43		)	(157		)	(53		)
Net gain on trading account securities	$	1,365		$	1,079		$	1,056