Securities (Tables)

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Summary of Amortized Cost and Fair Value of Available-for-sale and Held-to-maturity Securities

The amortized cost and fair value of securities available-for-sale and held-to-maturity at March 31, 2018 and December 31, 2017 are summarized as follows (in thousands):

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
March 31, 2018:
Securities available-for-sale:
U.S. Treasury securities	$	31,002	$	—	$	56	$	30,946
U.S. government agency securities	176,305		43		2,905		173,443
Mortgage-backed securities	1,302,274		4,393		29,394		1,277,273
State and municipal securities	1,206,777		3,890		18,202		1,192,465
Asset-backed securities	204,800		377		505		204,672
Corporate notes and other	81,860		868		903		81,825
	$	3,003,018	$	9,571	$	51,965	$	2,960,624
Securities held-to-maturity:
State and municipal securities	$	20,677	$	46	$	120	$	20,603
	$	20,677	$	46	$	120	$	20,603

December 31, 2017:
Securities available-for-sale:
U.S. Treasury securities	$	30,505	$	—	$	60	$	30,445
U.S. government agency securities	182,500		67		1,766		180,801
Mortgage-backed securities	1,270,625		5,318		12,124		1,263,819
State and municipal securities	774,949		12,251		2,588		784,612
Asset-backed securities	173,346		262		316		173,292
Corporate notes and other	81,615		1,346		647		82,314
	$	2,513,540	$	19,244	17,501		$	2,515,283
Securities held-to-maturity:
State and municipal securities	$	20,762	$	114	$	46	$	20,830
	$	20,762	$	114	$	46	$	20,830

Amortized Cost and Fair Value of Debt Securities by Contractual Maturity

The amortized cost and fair value of debt securities as of March 31, 2018 by contractual maturity are shown below. Actual maturities may differ from contractual maturities of mortgage- and asset-backed securities since the mortgages and assets underlying the securities may be called or prepaid with or without penalty. Therefore, these securities are not included in the maturity categories in the following summary (in thousands):

	Available-for-sale				Held-to-maturity
March 31, 2018:	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Due in one year or less	$	—	$	—	$	—	$	—
Due in one year to five years	96,941		97,087		7,496		7,489
Due in five years to ten years	177,898		177,231		10,385		10,316
Due after ten years	1,221,105		1,204,361		2,796		2,798
Mortgage-backed securities	1,302,274		1,277,273		—		—
Asset-backed securities	204,800		204,672		—		—
	$	3,003,018	$	2,960,624	$	20,677	$	20,603

Classification of Investments According to Term of Unrealized Losses of Less than Twelve Months or Twelve Months or Longer

At March 31, 2018 and December 31, 2017, the following investments had unrealized losses. The table below classifies these investments according to the term of the unrealized losses of less than twelve months or twelve months or longer (in thousands):

	Investments with an Unrealized Loss of less than 12 months				Investments with an Unrealized Loss of 12 months or longer				Total Investments with an Unrealized Loss
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
At March 31, 2018
U.S. Treasury securities	$	30,697	$	56	$	—	$	—	$	30,697	$	56
U.S. government agency securities	145,967		2,904		248		1		146,215		2,905
Mortgage-backed securities	779,194		17,392		297,735		12,002		1,076,929		29,394
State and municipal securities	863,010		15,987		47,894		2,335		910,904		18,322
Asset-backed securities	79,428		449		9,482		56		88,910		505
Corporate notes	31,880		754		11,697		149		43,577		903
Total temporarily-impaired securities	$	1,930,176	$	37,542	$	367,056	$	14,543	$	2,297,232	$	52,085
At December 31, 2017
U.S. Treasury securities	$	29,948	$	60	$	—	$	—	$	29,948	$	60
U.S. government agency securities	173,677		1,766		—		—		173,677		1,766
Mortgage-backed securities	607,408		5,042		285,561		7,082		892,969		12,124
State and municipal securities	115,403		1,408		50,083		1,226		165,486		2,634
Asset-backed securities	68,742		198		14,136		118		82,878		316
Corporate notes	22,168		547		11,944		100		34,112		647
Total temporarily-impaired securities	$	1,017,346	$	9,021	$	361,724	$	8,526	$	1,379,070	$	17,547