Securities (Tables)

9 Months Ended

Sep. 30, 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.

	September 30, 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	3,708		21		24		3,705		4,511		39		—		4,550
States and political subdivisions	1,437,164		36,991		2,556		1,471,599		1,994,710		16,821		6,335		2,005,196
Other	1,350		—		1		1,349		1,350		—		—		1,350
Total	$	1,442,222	$	37,012	$	2,581	$	1,476,653	$	2,250,460	$	18,622	$	6,335	$	2,262,747
Available for Sale
U.S. Treasury	$	3,452,882	$	23,796	$	3,050	$	3,473,628	$	4,003,692	$	24,984	$	8,945	$	4,019,731
Residential mortgage-backed securities	658,281		24,218		1,304		681,195		756,072		30,388		1,293		785,167
States and political subdivisions	5,898,098		130,142		40,501		5,987,739		5,403,918		50,101		98,134		5,355,885
Other	42,538		—		—		42,538		42,494		—		—		42,494
Total	$	10,051,799	$	178,156	$	44,855	$	10,185,100	$	10,206,176	$	105,473	$	108,372	$	10,203,277

Securities, with Unrealized Losses Segregated by Length of Impairment

As of September 30, 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
Residential mortgage-backed securities	$	2,212	$	24	$	—	$	—	$	2,212	$	24
States and political subdivisions	5,301		28		74,965		2,528		80,266		2,556
Other	1,349		1		—		—		1,349		1
Total	$	8,862	$	53	$	74,965	$	2,528	$	83,827	$	2,581
Available for Sale
U.S. Treasury	$	840,074	$	3,050	$	—	$	—	$	840,074	$	3,050
Residential mortgage-backed securities	75,441		618		19,458		686		94,899		1,304
States and political subdivisions	986,705		9,713		842,751		30,788		1,829,456		40,501
Total	$	1,902,220	$	13,381	$	862,209	$	31,474	$	2,764,429	$	44,855

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, 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	$	251,739	$	256,716	$	37,321	$	38,127
Due after one year through five years	116,604		121,451		4,056,709		4,085,795
Due after five years through ten years	411,074		420,160		385,649		399,538
Due after ten years	659,097		674,621		4,871,301		4,937,907
Residential mortgage-backed securities	3,708		3,705		658,281		681,195
Equity securities	—		—		42,538		42,538
Total	$	1,442,222	$	1,476,653	$	10,051,799	$	10,185,100

Sales of Securities Held-to-Maturity

Sales of securities held to maturity were as follows:

	Three Months Ended   September 30,				Nine Months Ended   September 30,
	2017		2016		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   September 30,						Nine Months Ended   September 30,
	2017			2016			2017			2016
Proceeds from sales	$	746,524		$	7,980,049		$	8,993,963		$	9,040,245
Gross realized gains	—			1			—			11,134
Gross realized losses	(4,867		)	(38		)	(4,917		)	(38		)
Tax (expense) benefit of securities gains/losses	1,703			13			1,721			(3,884		)

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,
	2017			2016			2017			2016
Premium amortization	$	(24,586	)	$	(22,762	)	$	(72,733	)	$	(67,321	)
Discount accretion	1,783			2,497			6,278			8,250
Net (premium amortization) discount accretion	$	(22,803	)	$	(20,265	)	$	(66,455	)	$	(59,071	)

Trading Account Securities, at Estimated Fair Value

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

	September 30, 2017		December 31, 2016
U.S. Treasury	$	18,814	$	16,594
States and political subdivisions	907		109
Total	$	19,721	$	16,703

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,
	2017			2016		2017			2016
Net gain on sales transactions	$	414		$	379	$	1,018		$	1,032
Net mark-to-market gains (losses)	(8		)	—		(51		)	(1		)
Net gain (loss) on trading account securities	$	406		$	379	$	967		$	1,031