Securities (Tables)

3 Months Ended

Mar. 31, 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.

	March 31, 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	$	3,503	$	12	$	57	$	3,458	$	3,610	$	15	$	38	$	3,587
States and political subdivisions	1,242,151		13,910		3,126		1,252,935		1,428,488		26,462		2,746		1,452,204
Other	1,500		—		7		1,493		—		—		—		—
Total	$	1,247,154	$	13,922	$	3,190	$	1,257,886	$	1,432,098	$	26,477	$	2,784	$	1,455,791
Available for Sale
U.S. Treasury	$	3,453,887	$	373	$	36,113	$	3,418,147	$	3,453,391	$	7,494	$	15,732	$	3,445,153
Residential mortgage-backed securities	636,498		16,012		6,815		645,695		648,288		19,048		2,250		665,086
States and political subdivisions	6,423,389		71,360		64,644		6,430,105		6,185,711		167,293		16,795		6,336,209
Other	42,585		—		—		42,585		42,561		—		—		42,561
Total	$	10,556,359	$	87,745	$	107,572	$	10,536,532	$	10,329,951	$	193,835	$	34,777	$	10,489,009

Securities, with Unrealized Losses Segregated by Length of Impairment

As of March 31, 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	$	1,120	$	23	$	1,516	$	34	$	2,636	$	57
States and political subdivisions	231,906		900		44,869		2,226		276,775		3,126
Other	1,493		7		—		—		1,493		7
Total	$	234,519	$	930	$	46,385	$	2,260	$	280,904	$	3,190
Available for Sale
U.S. Treasury	$	2,616,630	$	27,797	$	514,691	$	8,316	$	3,131,321	$	36,113
Residential mortgage-backed securities	219,091		4,508		45,424		2,307		264,515		6,815
States and political subdivisions	2,004,045		26,466		815,756		38,178		2,819,801		64,644
Total	$	4,839,766	$	58,771	$	1,375,871	$	48,801	$	6,215,637	$	107,572

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 March 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	$	105,054	$	106,904	$	118,442	$	119,061
Due after one year through five years	165,355		168,916		4,010,195		3,980,391
Due after five years through ten years	432,654		433,756		486,543		491,153
Due after ten years	540,588		544,852		5,262,096		5,257,647
Residential mortgage-backed securities	3,503		3,458		636,498		645,695
Equity securities	—		—		42,585		42,585
Total	$	1,247,154	$	1,257,886	$	10,556,359	$	10,536,532

Sales of Securities Available for Sale

Sales of securities available for sale were as follows:

	Three Months Ended   March 31,
	2018			2017
Proceeds from sales	$	2,984,867		$	—
Gross realized gains	—			—
Gross realized losses	(19		)	—
Tax (expense) benefit of securities gains/losses	4			—

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   March 31,
	2018			2017
Premium amortization	$	(26,036	)	$	(24,028	)
Discount accretion	1,776			2,390
Net (premium amortization) discount accretion	$	(24,260	)	$	(21,638	)

Trading Account Securities, at Estimated Fair Value

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

	March 31, 2018		December 31, 2017
U.S. Treasury	$	19,772	$	19,210
States and political subdivisions	—		1,888
Total	$	19,772	$	21,098

Net Gains and Losses on Trading Account Securities

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

	Three Months Ended   March 31,
	2018			2017
Net gain on sales transactions	$	505		$	311
Net mark-to-market gains (losses)	(36		)	13
Net gain (loss) on trading account securities	$	469		$	324