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Investment Securities (Tables)
 12 Months Ended
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]  
Schedule of Amortized Cost and Fair Values of Investment Securities
The following tables present the amortized cost and estimated fair values of investment securities, which were all classified as available for sale, as of December 31:
 
Amortized
Cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Estimated
Fair
Value
 
(in thousands)
2017
 
 
 
 
 
 
 
U.S. Government sponsored agency securities
$
5,962

 
$
2

 
$
(26
)
 
$
5,938

State and municipal securities
405,860

 
5,638

 
(2,549
)
 
408,949

Corporate debt securities
96,353

 
2,832

 
(1,876
)
 
97,309

Collateralized mortgage obligations
611,927

 
491

 
(9,795
)
 
602,623

Residential mortgage-backed securities
1,132,080

 
3,957

 
(15,241
)
 
1,120,796

Commercial mortgage-backed securities
215,351

 
—

 
(2,596
)
 
212,755

Auction rate securities
107,410

 
—

 
(8,742
)
 
98,668

   Total debt securities
2,574,943

 
12,920

 
(40,825
)
 
2,547,038

Equity securities
776

 
142

 
—

 
918

   Total
$
2,575,719

 
$
13,062

 
$
(40,825
)
 
$
2,547,956

 
 
 
 
 
 
 
 
2016
 
 
 
 
 
 
 
U.S. Government sponsored agency securities
$
132

 
$
2

 
$
—

 
$
134

State and municipal securities
405,274

 
2,043

 
(15,676
)
 
391,641

Corporate debt securities
112,016

 
1,978

 
(4,585
)
 
109,409

Collateralized mortgage obligations
604,095

 
1,943

 
(12,178
)
 
593,860

Residential mortgage-backed securities
1,328,192

 
6,546

 
(16,900
)
 
1,317,838

Commercial mortgage-backed securities
25,100

 
—

 
(537
)
 
24,563

Auction rate securities
107,215

 
—

 
(9,959
)
 
97,256

   Total debt securities
2,582,024

 
12,512

 
(59,835
)
 
2,534,701

Equity securities
12,231

 
12,295

 
—

 
24,526

   Total
$
2,594,255

 
$
24,807

 
$
(59,835
)
 
$
2,559,227
Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities
The amortized cost and estimated fair values of debt securities as of December 31, 2017, by contractual maturity, are shown in the following table. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.
 
Amortized
Cost
 
Estimated
Fair Value
 
(in thousands)
 
 
Due in one year or less
$
16,837

 
$
16,920

Due from one year to five years
33,191

 
33,565

Due from five years to ten years
112,181

 
113,164

Due after ten years
453,376

 
447,215

 
615,585

 
610,864

Residential mortgage-backed securities(1)
1,132,080

 
1,120,796

Commercial mortgage-backed securities(1)
215,351

 
212,755

Collateralized mortgage obligations (1)
611,927

 
602,623

Total debt securities
$
2,574,943

 
$
2,547,038



(1)
Maturities for mortgage-backed securities and collateralized mortgage obligations are dependent upon the interest rate environment and prepayments on the underlying loans.
Summary of Gains and Losses from Equity and Debt Securities, and Losses Recognized from Other-than-Temporary Impairment
The following table presents information related to gross gains and losses on the sales of equity and debt securities:
 
Gross
Realized
Gains
 
Gross
Realized
Losses
 
Net
Gains (Losses)
 
(in thousands)
2017:
 
 
 
 
 
Equity securities
$
13,558

 
$
—

 
$
13,558

Debt securities
315

 
(4,802
)
 
(4,487
)
Total
$
13,873

 
$
(4,802
)
 
$
9,071

2016:
 
 
 
 
 
Equity securities
$
2,005

 
$
(10
)
 
$
1,995

Debt securities
581

 
(26
)
 
555

Total
$
2,586

 
$
(36
)
 
$
2,550

2015:
 
 
 
 
 
Equity securities
$
6,496

 
$
(1
)
 
$
6,495

Debt securities
2,571

 
—

 
2,571

Total
$
9,067

 
$
(1
)
 
$
9,066
Summary of Cumulative Other-than-Temporary Impairment Charges Recognized in Earnings for Pooled Trust Preferred Securities Held
The following table presents a summary of the cumulative credit related other-than-temporary impairment charges, recognized as components of earnings, for debt securities held by the Corporation at December 31:
 
2017
 
2016
 
2015
 
(in thousands)
Balance of cumulative credit losses on debt securities, beginning of year
$
(11,510
)
 
$
(11,510
)
 
$
(16,242
)
Reductions for securities sold during the period
—

 
—

 
4,730

Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security
—

 
—

 
2

Balance of cumulative credit losses on debt securities, end of year
$
(11,510
)
 
$
(11,510
)
 
$
(11,510
)
Gross Unrealized Losses and Fair Values of Investments by Category and Length of Time in Continuous Unrealized Loss Position
The following table presents the gross unrealized losses and estimated fair values of investments, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, as of December 31, 2017. There were no gross unrealized losses on equity securities as of December 31, 2017.

 
Less Than 12 months
 
12 Months or Longer
 
Total
 
Estimated
Fair Value
 
Unrealized
Losses
 
Estimated
Fair Value
 
Unrealized
Losses
 
Estimated
Fair Value
 
Unrealized
Losses
 
(in thousands)
U.S. Government sponsored agency securities
$
5,830

 
$
(26
)
 
$
—

 
$
—

 
$
5,830

 
$
(26
)
State and municipal securities
11,650

 
(50
)
 
118,297

 
(2,499
)
 
129,947

 
(2,549
)
Corporate debt securities
4,544

 
(48
)
 
32,163

 
(1,828
)
 
36,707

 
(1,876
)
Collateralized mortgage obligations
303,932

 
(2,408
)
 
187,690

 
(7,387
)
 
491,622

 
(9,795
)
Residential mortgage-backed securities
511,378

 
(4,348
)
 
500,375

 
(10,893
)
 
1,011,753

 
(15,241
)
Commercial mortgage-backed securities
190,985

 
(2,118
)
 
21,770

 
(478
)
 
212,755

 
(2,596
)
Auction rate securities
—

 
—

 
98,668

 
(8,742
)
 
98,668

 
(8,742
)
Total
$
1,028,319

 
$
(8,998
)
 
$
958,963

 
$
(31,827
)
 
$
1,987,282

 
$
(40,825
)


For comparative purposes, the following table presents gross unrealized losses and the estimated fair value of investments, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, at December 31, 2016.
 
Less Than 12 months
 
12 Months or Longer
 
Total
 
Estimated
Fair Value
 
Unrealized
Losses
 
Estimated
Fair Value
 
Unrealized
Losses
 
Estimated
Fair Value
 
Unrealized
Losses
 
(in thousands)
State and municipal securities
$
247,509

 
$
(15,676
)
 
$
—

 
$
—

 
$
247,509

 
$
(15,676
)
Corporate debt securities
11,922

 
(110
)
 
34,629

 
(4,475
)
 
46,551

 
(4,585
)
Collateralized mortgage obligations
166,905

 
(3,899
)
 
258,237

 
(8,279
)
 
425,142

 
(12,178
)
Mortgage-backed securities
1,137,510

 
(17,437
)
 
—

 
—

 
1,137,510

 
(17,437
)
Auction rate securities
—

 
—

 
97,256

 
(9,959
)
 
97,256

 
(9,959
)
Total
$
1,563,846

 
$
(37,122
)
 
$
390,122

 
$
(22,713
)
 
$
1,953,968

 
$
(59,835
)
Summary of Amortized Cost and Fair Values of Corporate Debt Securities
The majority of the Corporation’s available for sale corporate debt securities are issued by financial institutions. The following table presents the amortized cost and estimated fair values of corporate debt securities as of December 31:
 
2017
 
2016
 
Amortized
Cost
 
Estimated
Fair Value
 
Amortized
Cost
 
Estimated
Fair Value
 
(in thousands)
Single-issuer trust preferred securities
$
31,335

 
$
30,703

 
$
43,746

 
$
39,829

Subordinated debt
49,013

 
49,533

 
46,231

 
46,723

Senior notes
12,031

 
12,392

 
18,037

 
18,433

Pooled trust preferred securities
—

 
707

 
—

 
422

Corporate debt securities issued by financial institutions
92,379

 
93,335

 
108,014

 
105,407

Other corporate debt securities
3,974

 
3,974

 
4,002

 
4,002

Available for sale corporate debt securities
$
96,353

 
$
97,309

 
$
112,016

 
$
109,409