Securities Available for Sale and Held to Maturity (Tables)

12 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Amortized Cost and Estimated Fair Value of Available for Sale and Held to Maturity Securities

The following tables are a summary of the amortized cost and estimated fair value of securities available for sale and held to maturity at December 31, 2018 and 2017 ($ in thousands):

	Securities Available for Sale									Securities Held to Maturity
			Gross		Gross			Estimated				Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair		Amortized		Unrealized		Unrealized			Fair
December 31, 2018	Cost		Gains		Losses			Value		Cost		Gains		Losses			Value
U.S. Government agency obligations	$	31,235	$	109	$	(1,009	)	$	30,335	$	3,736	$	78	$	—		$	3,814
Obligations of states and political    subdivisions		50,503		200		(27	)		50,676		35,783		255		(139	)		35,899
Mortgage-backed securities
Residential mortgage pass-through    securities
Guaranteed by GNMA		69,648		147		(2,301	)		67,494		12,090		45		(257	)		11,878
Issued by FNMA and FHLMC		685,520		127		(18,963	)		666,684		115,133		43		(2,887	)		112,289
Other residential mortgage-backed    securities
Issued or guaranteed by FNMA,    FHLMC or GNMA		830,129		67		(18,595	)		811,601		578,827		189		(15,441	)		563,575
Commercial mortgage-backed    securities
Issued or guaranteed by FNMA,    FHLMC or GNMA		187,494		191		(2,662	)		185,023		164,074		299		(2,095	)		162,278
Total	$	1,854,529	$	841	$	(43,557	)	$	1,811,813	$	909,643	$	909	$	(20,819	)	$	889,733
December 31, 2017
U.S. Government agency obligations	$	45,763	$	322	$	(800	)	$	45,285	$	3,692	$	182	$	—		$	3,874
Obligations of states and political    subdivisions		78,433		850		(54	)		79,229		46,039		1,044		(59	)		47,024
Mortgage-backed securities
Residential mortgage pass-through    securities
Guaranteed by GNMA		66,634		215		(1,103	)		65,746		13,539		207		(73	)		13,673
Issued by FNMA and FHLMC		824,872		827		(11,249	)		814,450		133,975		210		(1,559	)		132,626
Other residential mortgage-backed    securities
Issued or guaranteed by FNMA,    FHLMC or GNMA		1,028,176		1,808		(13,194	)		1,016,790		678,926		1,209		(11,065	)		669,070
Commercial mortgage-backed    securities
Issued or guaranteed by FNMA,    FHLMC or GNMA		218,252		426		(1,543	)		217,135		180,315		1,102		(1,437	)		179,980
Total	$	2,262,130	$	4,448	$	(27,943	)	$	2,238,635	$	1,056,486	$	3,954	$	(14,193	)	$	1,046,247

Securities with Gross Unrealized Losses, Segregated by Length of Impairment

The table below includes securities with gross unrealized losses segregated by length of impairment at December 31, 2018 and 2017 ($ in thousands):

	Less than 12 Months					12 Months or More					Total
			Gross					Gross					Gross
	Estimated		Unrealized			Estimated		Unrealized			Estimated		Unrealized
December 31, 2018	Fair Value		Losses			Fair Value		Losses			Fair Value		Losses
U.S. Government agency obligations	$	—	$	—		$	25,045	$	(1,009	)	$	25,045	$	(1,009	)
Obligations of states and political    subdivisions		4,954		(9	)		12,802		(157	)		17,756		(166	)
Mortgage-backed securities
Residential mortgage pass-through    securities
Guaranteed by GNMA		9,163		(54	)		61,141		(2,504	)		70,304		(2,558	)
Issued by FNMA and FHLMC		31,931		(172	)		731,749		(21,678	)		763,680		(21,850	)
Other residential mortgage-backed    securities
Issued or guaranteed by FNMA,    FHLMC or GNMA		46,643		(110	)		1,296,221		(33,926	)		1,342,864		(34,036	)
Commercial mortgage-backed    securities
Issued or guaranteed by FNMA,    FHLMC or GNMA		5,497		(37	)		272,789		(4,720	)		278,286		(4,757	)
Total	$	98,188	$	(382	)	$	2,399,747	$	(63,994	)	$	2,497,935	$	(64,376	)
December 31, 2017
U.S. Government agency obligations	$	5,214	$	(113	)	$	29,432	$	(687	)	$	34,646	$	(800	)
Obligations of states and political    subdivisions		19,345		(80	)		3,874		(33	)		23,219		(113	)
Mortgage-backed securities
Residential mortgage pass-through    securities
Guaranteed by GNMA		37,304		(351	)		29,446		(825	)		66,750		(1,176	)
Issued by FNMA and FHLMC		506,410		(4,219	)		369,060		(8,589	)		875,470		(12,808	)
Other residential mortgage-backed    securities
Issued or guaranteed by FNMA,    FHLMC or GNMA		755,013		(7,668	)		534,955		(16,591	)		1,289,968		(24,259	)
Commercial mortgage-backed    securities
Issued or guaranteed by FNMA,    FHLMC or GNMA		230,898		(1,719	)		55,288		(1,261	)		286,186		(2,980	)
Total	$	1,554,184	$	(14,150	)	$	1,022,055	$	(27,986	)	$	2,576,239	$	(42,136	)

Gains and Losses as a Result of Calls and Disposition of Securities

For the periods presented, gross realized gains or losses as a result of calls and dispositions of securities, as well as any associated proceeds, were as follows ($ in thousands):

	Years Ended December 31,
Available for Sale	2018		2017			2016
Proceeds from calls and sales of securities	$	—	$	27,682		$	24,693
Gross realized gains		—		16			32
Gross realized losses		—		(1	)		(342	)

Contractual Maturities of Available for Sale and Held to Maturity Securities

The amortized cost and estimated fair value of securities available for sale and held to maturity at December 31, 2018, by contractual maturity, are shown below ($ in thousands).  Expected maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.

	Securities				Securities
	Available for Sale				Held to Maturity
	Amortized		Estimated		Amortized		Estimated
	Cost		Fair Value		Cost		Fair Value
Due in one year or less	$	24,449	$	24,475	$	170	$	170
Due after one year through five years		27,271		27,481		35,073		35,193
Due after five years through ten years		3,056		2,989		4,276		4,350
Due after ten years		26,962		26,066		—		—
		81,738		81,011		39,519		39,713
Mortgage-backed securities		1,772,791		1,730,802		870,124		850,020
Total	$	1,854,529	$	1,811,813	$	909,643	$	889,733