Securities (Tables)

12 Months Ended

Dec. 31, 2016

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:

	2016								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	$	249,889	$	1,762	$	—	$	251,651	$	249,441	$	7,776	$	—	$	257,217
Residential mortgage-backed securities	4,511		39		—		4,550		6,456		63		4		6,515
States and political subdivisions	1,994,710		16,821		6,335		2,005,196		2,405,762		46,003		6,149		2,445,616
Other	1,350		—		—		1,350		1,350		—		13		1,337
Total	$	2,250,460	$	18,622	$	6,335	$	2,262,747	$	2,663,009	$	53,842	$	6,166	$	2,710,685
Available for Sale:
U.S. Treasury	$	4,003,692	$	24,984	$	8,945	$	4,019,731	$	3,980,986	$	22,041	$	8,507	$	3,994,520
Residential mortgage-backed securities	756,072		30,388		1,293		785,167		1,000,024		42,142		734		1,041,432
States and political subdivisions	5,403,918		50,101		98,134		5,355,885		3,996,113		133,305		1,459		4,127,959
Other	42,494		—		—		42,494		42,447		—		—		42,447
Total	$	10,206,176	$	105,473	$	108,372	$	10,203,277	$	9,019,570	$	197,488	$	10,700	$	9,206,358

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
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

	Less than 12 Months				More than 12 Months				Total
	Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses
2015
Held to Maturity:
Residential mortgage-backed securities	$	900	$	4	$	—	$	—	$	900	$	4
States and political subdivisions	146,854		1,325		202,423		4,824		349,277		6,149
Other	1,337		13		—		—		1,337		13
Total	$	149,091	$	1,342	$	202,423	$	4,824	$	351,514	$	6,166
Available for Sale:
U.S. Treasury	$	886,087	$	8,507	$	—	$	—	$	886,087	$	8,507
Residential mortgage-backed securities	21,392		212		17,781		522		39,173		734
States and political subdivisions	120,782		1,237		18,485		222		139,267		1,459
Total	$	1,028,261	$	9,956	$	36,266	$	744	$	1,064,527	$	10,700

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, 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	$	738,493	$	745,518	$	90,632	$	91,753
Due after one year through five years	297,244		311,767		4,632,290		4,649,251
Due after five years through ten years	312,357		311,980		348,731		357,762
Due after ten years	897,855		888,932		4,335,957		4,276,850
Residential mortgage-backed securities	4,511		4,550		756,072		785,167
Equity securities	—		—		42,494		42,494
Total	$	2,250,460	$	2,262,747	$	10,206,176	$	10,203,277

Held-to-maturity Securities [Table Text Block]

Sales of securities held to maturity were as follows:

	2016			2015		2014
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:

	2016			2015			2014
Proceeds from sales	$	14,847,380		$	12,683,169		$	12,151,287
Gross realized gains	13,289			228			39
Gross realized losses	(2,059		)	(159		)	(1		)
Tax expense related to securities gains/losses	(3,931		)	(24		)	(13		)

Premium Amortization and Discount Accretion Included in Income on Securities

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

	2016			2015			2014
Premium amortization	$	(90,782	)	$	(84,467	)	$	(68,070	)
Discount accretion	11,077			10,682			6,802
Net (premium amortization) discount accretion	$	(79,705	)	$	(73,785	)	$	(61,268	)

Year End Trading Account Securities, at Estimated Fair Value

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

	2016		2015
U.S. Treasury	$	16,594	$	16,443
States and political subdivisions	109		136
Total	$	16,703	$	16,579

Net Gains and Losses on Trading Account Securities

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

	2016			2015			2014
Net gain on sales transactions	$	1,236		$	1,109		$	829
Net mark-to-market gains (losses)	(157		)	(53		)	—
Net gain on trading account securities	$	1,079		$	1,056		$	829