Securities (Tables)

3 Months Ended

Mar. 31, 2017

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, 2017								December 31, 2016
	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
Residential mortgage-backed securities	4,313		30		—		4,343		4,511		39		—		4,550
States and political subdivisions	1,634,592		27,531		3,039		1,659,084		1,994,710		16,821		6,335		2,005,196
Other	1,350		—		4		1,346		1,350		—		—		1,350
Total	$	1,640,255	$	27,561	$	3,043	$	1,664,773	$	2,250,460	$	18,622	$	6,335	$	2,262,747
Available for Sale
U.S. Treasury	$	4,203,543	$	25,386	$	7,698	$	4,221,231	$	4,003,692	$	24,984	$	8,945	$	4,019,731
Residential mortgage-backed securities	711,040		27,305		1,426		736,919		756,072		30,388		1,293		785,167
States and political subdivisions	5,624,493		70,205		82,860		5,611,838		5,403,918		50,101		98,134		5,355,885
Other	42,505		—		—		42,505		42,494		—		—		42,494
Total	$	10,581,581	$	122,896	$	91,984	$	10,612,493	$	10,206,176	$	105,473	$	108,372	$	10,203,277

Securities, with Unrealized Losses Segregated by Length of Impairment

As of March 31, 2017, 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
States and political subdivisions	$	27,969	$	249	$	120,214	$	2,790	$	148,183	$	3,039
Other	1,346		4		—		—		1,346		4
Total	$	29,315	$	253	$	120,214	$	2,790	$	149,529	$	3,043
Available for Sale
U.S. Treasury	$	1,621,498	$	7,698	$	—	$	—	$	1,621,498	$	7,698
Residential mortgage-backed securities	61,669		1,166		$	6,200	260		67,869		1,426
States and political subdivisions	2,182,114		82,860		—		—		2,182,114		82,860
Total	$	3,865,281	$	91,724	$	6,200	$	260	$	3,871,481	$	91,984

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, 2017 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	$	327,143	$	335,353	$	50,830	$	51,606
Due after one year through five years	166,880		174,676		4,793,171		4,819,082
Due after five years through ten years	349,195		353,119		366,528		376,888
Due after ten years	792,724		797,282		4,617,507		4,585,493
Residential mortgage-backed securities	4,313		4,343		711,040		736,919
Equity securities	—		—		42,505		42,505
Total	$	1,640,255	$	1,664,773	$	10,581,581	$	10,612,493

Sales of Securities Held-to-Maturity

Sales of securities held to maturity were as follows:

	Three Months Ended   March 31,
	2017		2016
Proceeds from sales	$	—	$	135,610
Amortized cost	—		131,840
Gross realized gains	—		3,770
Gross realized losses	—		—
Tax (expense) benefit of securities gains/losses	—		(1,319		)

Sales of Securities Available for Sale

Sales of securities available for sale were as follows:

	Three Months Ended   March 31,
	2017		2016
Proceeds from sales	$	—	$	1,060,196
Gross realized gains	—		11,133
Gross realized losses	—		—
Tax (expense) benefit of securities gains/losses	—		(3,897		)

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,
	2017			2016
Premium amortization	$	(24,028	)	$	(22,340	)
Discount accretion	2,390			2,615
Net (premium amortization) discount accretion	$	(21,638	)	$	(19,725	)

Trading Account Securities, at Estimated Fair Value

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

	March 31, 2017		December 31, 2016
U.S. Treasury	$	17,094	$	16,594
States and political subdivisions	—		109
Total	$	17,094	$	16,703

Net Gains and Losses on Trading Account Securities

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

	Three Months Ended   March 31,
	2017		2016
Net gain on sales transactions	$	311	$	302
Net mark-to-market gains (losses)	13		1
Net gain (loss) on trading account securities	$	324	$	303