Securities (Tables)

12 Months Ended

Dec. 31, 2018

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:

	2018								2017
	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:
Residential mortgage-backed securities	$	2,737	$	8	$	85	$	2,660	$	3,610	$	15	$	38	$	3,587
States and political subdivisions	1,101,820		11,525		552		1,112,793		1,428,488		26,462		2,746		1,452,204
Other	1,500		—		—		1,500		—		—		—		—
Total	$	1,106,057	$	11,533	$	637	$	1,116,953	$	1,432,098	$	26,477	$	2,784	$	1,455,791
Available for Sale:
U.S. Treasury	$	3,455,417	$	1,772	$	29,500	$	3,427,689	$	3,453,391	$	7,494	$	15,732	$	3,445,153
Residential mortgage-backed securities	823,208		13,079		6,547		829,740		648,288		19,048		2,250		665,086
States and political subdivisions	7,089,132		70,760		72,690		7,087,202		6,185,711		167,293		16,795		6,336,209
Other	42,690		—		—		42,690		42,561		—		—		42,561
Total	$	11,410,447	$	85,611	$	108,737	$	11,387,321	$	10,329,951	$	193,835	$	34,777	$	10,489,009

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
2018
Held to Maturity:
Residential mortgage-backed securities	$	—	$	—	$	2,034	$	85	$	2,034	$	85
States and political subdivisions	205,686		541		5,952		11		211,638		552
Total	$	205,686	$	541	$	7,986	$	96	$	213,672	$	637
Available for Sale:
U.S. Treasury	$	—	$	—	$	3,139,639	$	29,500	$	3,139,639	$	29,500
Residential mortgage-backed securities	152,682		205		213,982		6,342		366,664		6,547
States and political subdivisions	1,136,322		7,026		2,058,048		65,664		3,194,370		72,690
Total	$	1,289,004	$	7,231	$	5,411,669	$	101,506	$	6,700,673	$	108,737

	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

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, 2018 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	$	72,476	$	73,139	$	2,235,652	$	2,215,867
Due after one year through five years	122,796		124,738		1,858,631		1,853,056
Due after five years through ten years	483,934		487,202		486,008		492,435
Due after ten years	424,114		429,214		5,964,258		5,953,533
Residential mortgage-backed securities	2,737		2,660		823,208		829,740
Equity securities	—		—		42,690		42,690
Total	$	1,106,057	$	1,116,953	$	11,410,447	$	11,387,321

Debt Securities, Held-to-maturity [Table Text Block]

Sales of securities held to maturity were as follows:

	2018		2017		2016
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:

	2018			2017			2016
Proceeds from sales	$	16,806,062		$	11,963,359		$	14,847,380
Gross realized gains	3			1			13,289
Gross realized losses	(159		)	(4,942		)	(2,059		)
Tax benefit (expense) related to securities gains/losses	33			1,729			(3,931		)

Premium Amortization and Discount Accretion Included in Income on Securities

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

	2018			2017			2016
Premium amortization	$	(108,483	)	$	(97,841	)	$	(90,782	)
Discount accretion	7,955			7,908			11,077
Net (premium amortization) discount accretion	$	(100,528	)	$	(89,933	)	$	(79,705	)

Year End Trading Account Securities, at Estimated Fair Value

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

	2018		2017
U.S. Treasury	$	21,928	$	19,210
States and political subdivisions	2,158		1,888
Total	$	24,086	$	21,098

Net Gains and Losses on Trading Account Securities

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

	2018		2017			2016
Net gain on sales transactions	$	1,816	$	1,408		$	1,236
Net mark-to-market gains (losses)	105		(43		)	(157		)
Net gain on trading account securities	$	1,921	$	1,365		$	1,079