Securities (Tables)

6 Months Ended

Jun. 30, 2015

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, 2015								December 31, 2014
	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	$	249,222	$	12,575	$	—	$	261,797	$	249,009	$	14,604	$	—	$	263,613
Residential mortgage-backed securities	6,960		60		10		7,010		8,012		92		—		8,104
States and political subdivisions	2,538,144		24,278		18,589		2,543,833		2,668,115		34,243		9,035		2,693,323
Other	1,350		—		—		1,350		1,350		—		—		1,350
Total	$	2,795,676	$	36,913	$	18,599	$	2,813,990	$	2,926,486	$	48,939	$	9,035	$	2,966,390
Available for Sale
U.S. Treasury	$	3,712,335	$	35,008	$	4,794	$	3,742,549	$	3,783,899	$	30,594	$	3,241	$	3,811,252
U.S. Government agencies/corporations	10,002		—		—		10,002		—		—		—		—
Residential mortgage-backed securities	1,165,458		56,322		873		1,220,907		1,331,114		68,027		417		1,398,724
States and political subdivisions	3,593,042		63,178		28,503		3,627,717		3,104,563		104,500		156		3,208,907
Other	42,411		—		—		42,411		42,371		—		—		42,371
Total	$	8,523,248	$	154,508	$	34,170	$	8,643,586	$	8,261,947	$	203,121	$	3,814	$	8,461,254

Securities, with Unrealized Losses Segregated by Length of Impairment

As of June 30, 2015, 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	$	5,123	$	10	$	—	$	—	$	5,123	$	10
States and political subdivisions	$	845,645	$	11,126	$	251,968	$	7,463	$	1,097,613	$	18,589
Total	$	850,768	$	11,136	$	251,968	$	7,463	$	1,102,736	$	18,599
Available for Sale
U.S. Treasury	$	522,998	$	4,794	$	—	$	—	$	522,998	$	4,794
Residential mortgage-backed securities	32,437		393		11,832		480		44,269		873
States and political subdivisions	1,365,860		28,503		—		—		1,365,860		28,503
Total	$	1,921,295	$	33,690	$	11,832	$	480	$	1,933,127	$	34,170

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, 2015 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	$	218,842	$	222,691	$	17,396	$	17,668
Due after one year through five years	748,446		797,614		3,315,042		3,343,291
Due after five years through ten years	187,343		186,569		1,543,263		1,571,169
Due after ten years	1,634,085		1,600,106		2,439,678		2,448,140
Residential mortgage-backed securities	6,960		7,010		1,165,458		1,220,907
Equity securities	—		—		42,411		42,411
Total	$	2,795,676	$	2,813,990	$	8,523,248	$	8,643,586

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,
	2015		2014			2015			2014
Proceeds from sales	$	—	$	2,028		$	223,987		$	2,028
Gross realized gains	—		3			228			3
Gross realized losses	—		(1		)	—			(1		)
Tax (expense) benefit of securities gains/losses	—		(1		)	(80		)	(1		)

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,
	2015			2014			2015			2014
Premium amortization	$	(20,832	)	$	(15,995	)	$	(40,838	)	$	(31,391	)
Discount accretion	2,629			1,446			5,327			3,032
Net (premium amortization) discount accretion	$	(18,203	)	$	(14,549	)	$	(35,511	)	$	(28,359	)

Trading Account Securities, at Estimated Fair Value

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

	June 30, 2015		December 31, 2014
U.S. Treasury	$	15,639	$	15,339
States and political subdivisions	957		87
Total	$	16,596	$	15,426

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,
	2015			2014		2015			2014
Net gain on sales transactions	$	278		$	253	$	558		$	493
Net mark-to-market gains (losses)	(32		)	—		(46		)	(3		)
Net gain (loss) on trading account securities	$	246		$	253	$	512		$	490