Securities (Tables)

6 Months Ended

Jun. 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.

	June 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	$	3,317	$	9	$	73	$	3,253	$	3,610	$	15	$	38	$	3,587
States and political subdivisions	1,231,694		13,100		2,394		1,242,400		1,428,488		26,462		2,746		1,452,204
Other	1,500		—		10		1,490		—		—		—		—
Total	$	1,236,511	$	13,109	$	2,477	$	1,247,143	$	1,432,098	$	26,477	$	2,784	$	1,455,791
Available for Sale
U.S. Treasury	$	3,454,387	$	—	$	44,306	$	3,410,081	$	3,453,391	$	7,494	$	15,732	$	3,445,153
Residential mortgage-backed securities	625,897		13,540		7,779		631,658		648,288		19,048		2,250		665,086
States and political subdivisions	6,626,495		71,701		64,807		6,633,389		6,185,711		167,293		16,795		6,336,209
Other	42,615		—		—		42,615		42,561		—		—		42,561
Total	$	10,749,394	$	85,241	$	116,892	$	10,717,743	$	10,329,951	$	193,835	$	34,777	$	10,489,009

Securities, with Unrealized Losses Segregated by Length of Impairment

As of June 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	$	1,103	$	25	$	1,392	$	48	$	2,495	$	73
States and political subdivisions	232,707		633		44,490		1,761		277,197		2,394
Other	1,490		10		—		—		1,490		10
Total	$	235,300	$	668	$	45,882	$	1,809	$	281,182	$	2,477
Available for Sale
U.S. Treasury	$	2,841,825	$	34,887	$	568,256	$	9,419	$	3,410,081	$	44,306
Residential mortgage-backed securities	238,548		4,687		58,923		3,092		297,471		7,779
States and political subdivisions	1,907,610		25,535		813,864		39,272		2,721,474		64,807
Total	$	4,987,983	$	65,109	$	1,441,043	$	51,783	$	6,429,026	$	116,892

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, 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	$	138,403	$	140,001	$	400,123	$	398,992
Due after one year through five years	134,389		136,874		3,739,862		3,705,611
Due after five years through ten years	448,447		450,758		480,038		481,384
Due after ten years	511,955		516,257		5,460,859		5,457,483
Residential mortgage-backed securities	3,317		3,253		625,897		631,658
Equity securities	—		—		42,615		42,615
Total	$	1,236,511	$	1,247,143	$	10,749,394	$	10,717,743

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,
	2018			2017			2018			2017
Proceeds from sales	$	7,905,521		$	8,247,439		$	10,890,388		$	8,247,439
Gross realized gains	3			—			3			—
Gross realized losses	(63		)	(50		)	(82		)	(50		)
Tax (expense) benefit of securities gains/losses	13			18			17			18

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,
	2018			2017			2018			2017
Premium amortization	$	(26,689	)	$	(24,119	)	$	(52,723	)	$	(48,147	)
Discount accretion	2,010			2,105			3,787			4,495
Net (premium amortization) discount accretion	$	(24,679	)	$	(22,014	)	$	(48,936	)	$	(43,652	)

Trading Account Securities, at Estimated Fair Value

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

	June 30, 2018		December 31, 2017
U.S. Treasury	$	20,755	$	19,210
States and political subdivisions	579		1,888
Total	$	21,334	$	21,098

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,
	2018		2017			2018			2017
Net gain on sales transactions	$	434	$	293		$	939		$	604
Net mark-to-market gains (losses)	23		(56		)	(13		)	(43		)
Net gain (loss) on trading account securities	$	457	$	237		$	926		$	561