Securities (Tables)

9 Months Ended

Sep. 30, 2018

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.

	September 30, 2018								December 31, 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	$	3,040	$	8	$	94	$	2,954	$	3,610	$	15	$	38	$	3,587
States and political subdivisions	1,107,222		7,232		2,057		1,112,397		1,428,488		26,462		2,746		1,452,204
Other	1,500		—		6		1,494		—		—		—		—
Total	$	1,111,762	$	7,240	$	2,157	$	1,116,845	$	1,432,098	$	26,477	$	2,784	$	1,455,791
Available for Sale
U.S. Treasury	$	3,454,899	$	—	$	52,020	$	3,402,879	$	3,453,391	$	7,494	$	15,732	$	3,445,153
Residential mortgage-backed securities	756,649		11,254		11,532		756,371		648,288		19,048		2,250		665,086
States and political subdivisions	7,052,156		40,759		116,225		6,976,690		6,185,711		167,293		16,795		6,336,209
Other	42,651		—		—		42,651		42,561		—		—		42,561
Total	$	11,306,355	$	52,013	$	179,777	$	11,178,591	$	10,329,951	$	193,835	$	34,777	$	10,489,009

Securities, with Unrealized Losses Segregated by Length of Impairment

As of September 30, 2018, 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	$	413	$	19	$	1,850	$	75	$	2,263	$	94
States and political subdivisions	403,965		2,057		—		—		403,965		2,057
Other	1,494		6		—		—		1,494		6
Total	$	405,872	$	2,082	$	1,850	$	75	$	407,722	$	2,157
Available for Sale
U.S. Treasury	$	2,295,611	$	34,315	$	1,107,268	$	17,705	$	3,402,879	$	52,020
Residential mortgage-backed securities	294,839		5,979		98,757		5,553		393,596		11,532
States and political subdivisions	3,585,212		65,581		810,732		50,644		4,395,944		116,225
Total	$	6,175,662	$	105,875	$	2,016,757	$	73,902	$	8,192,419	$	179,777

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 September 30, 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	$	56,511	$	57,181	$	858,235	$	851,163
Due after one year through five years	131,987		133,521		3,243,532		3,204,804
Due after five years through ten years	492,460		492,354		489,960		487,536
Due after ten years	427,764		430,835		5,915,328		5,836,066
Residential mortgage-backed securities	3,040		2,954		756,649		756,371
Equity securities	—		—		42,651		42,651
Total	$	1,111,762	$	1,116,845	$	11,306,355	$	11,178,591

Sales of Securities Available for Sale

Sales of securities available for sale were as follows:

	Three Months Ended   September 30,						Nine Months Ended   September 30,
	2018			2017			2018			2017
Proceeds from sales	$	2,948,178		$	746,524		$	13,838,566		$	8,993,963
Gross realized gains	—			—			3			—
Gross realized losses	(34		)	(4,867		)	(116		)	(4,917		)
Tax (expense) benefit of securities gains/losses	7			1,703			24			1,721

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   September 30,						Nine Months Ended   September 30,
	2018			2017			2018			2017
Premium amortization	$	(27,381	)	$	(24,586	)	$	(80,104	)	$	(72,733	)
Discount accretion	2,569			1,783			6,356			6,278
Net (premium amortization) discount accretion	$	(24,812	)	$	(22,803	)	$	(73,748	)	$	(66,455	)

Trading Account Securities, at Estimated Fair Value

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

	September 30, 2018		December 31, 2017
U.S. Treasury	$	21,804	$	19,210
States and political subdivisions	434		1,888
Total	$	22,238	$	21,098

Net Gains and Losses on Trading Account Securities

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

	Three Months Ended   September 30,						Nine Months Ended   September 30,
	2018			2017			2018			2017
Net gain on sales transactions	$	465		$	414		$	1,404		$	1,018
Net mark-to-market gains (losses)	(8		)	(8		)	(21		)	(51		)
Net gain (loss) on trading account securities	$	457		$	406		$	1,383		$	967