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Investment Securities | 6 Months Ended
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Jun. 30, 2017
Investments, Debt and Equity Securities [Abstract] |
Investment Securities | Investment Securities The following table presents the amortized cost and estimated fair values of investment securities, which were all classified as available for sale:
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Value
| (in thousands)
June 30, 2017 |  |  |  |  |  |  |
U.S. Government sponsored agency securities | $ | 5,960 |  |  | $ | 94 |  |  | $ | — |  |  | $ | 6,054 |
State and municipal securities | 405,035 |  |  | 3,726 |  |  | (7,125 | ) |  | 401,636 |
Corporate debt securities | 88,887 |  |  | 2,531 |  |  | (2,426 | ) |  | 88,992 |
Collateralized mortgage obligations | 546,847 |  |  | 1,399 |  |  | (9,739 | ) |  | 538,507 |
Residential mortgage-backed securities | 1,218,257 |  |  | 5,782 |  |  | (10,974 | ) |  | 1,213,065 |
Commercial mortgage-backed securities | 121,012 |  |  | 390 |  |  | (335 | ) |  | 121,067 |
Auction rate securities | 107,410 |  |  | — |  |  | (9,487 | ) |  | 97,923 |
Total debt securities | 2,493,408 |  |  | 13,922 |  |  | (40,086 | ) |  | 2,467,244 |
Equity securities | 10,487 |  |  | 10,968 |  |  | — |  |  | 21,455 |
Total | $ | 2,503,895 |  |  | $ | 24,890 |  |  | $ | (40,086 | ) |  | $ | 2,488,699 |
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Value
| (in thousands)
December 31, 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 |
Securities carried at $1.7 billion and $1.8 billion as of June 30, 2017 and December 31, 2016, respectively, were pledged as collateral to secure public and trust deposits and customer repurchase agreements. Equity securities include common stocks of publicly traded financial institutions (estimated fair value of $20.5 million at June 30, 2017 and $23.5 million at December 31, 2016) and other equity investments (estimated fair value of $1.0 million at both June 30, 2017 and December 31, 2016). As of June 30, 2017, the financial institutions stock portfolio had a cost basis of $9.7 million and an estimated fair value of $20.5 million, including an investment in a single financial institution with a cost basis of $4.3 million and an estimated fair value of $8.8 million. The estimated fair value of this investment accounted for 42.9% of the estimated fair value of the Corporation's investments in the common stocks of publicly traded financial institutions. No other investment in a single financial institution in the financial institutions stock portfolio exceeded 10% of the portfolio's estimated fair value. The amortized cost and estimated fair values of debt securities as of June 30, 2017, by contractual maturity, are shown in the following table. Actual maturities may differ from contractual maturities as certain investment securities are subject to call or prepayment with or without call or prepayment penalties.
|  | Amortized Cost |  | Estimated Fair Value
| (in thousands)
Due in one year or less |  | $ | 20,103 |  |  | $ | 20,336 |
Due from one year to five years |  | 27,967 |  |  | 28,335 |
Due from five years to ten years |  | 111,883 |  |  | 113,050 |
Due after ten years |  | 447,339 |  |  | 432,884 |
|  | 607,292 |  |  | 594,605 |
Residential mortgage-backed securities |  | 1,218,257 |  |  | 1,213,065 |
Commercial mortgage-backed securities |  | 121,012 |  |  | 121,067 |
Collateralized mortgage obligations |  | 546,847 |  |  | 538,507 |
Total debt securities |  | $ | 2,493,408 |  |  | $ | 2,467,244 |
The following table presents information related to the gross realized gains and losses on the sales of equity and debt securities:
| Gross Realized Gains |  | Gross Realized Losses |  | Net Gains (Losses)
Three months ended June 30, 2017 | (in thousands)
Equity securities | $ | 1,305 |  |  | $ | — |  |  | $ | 1,305 |
Debt securities | 145 |  |  | (14 | ) |  | 131 |
Total | $ | 1,450 |  |  | $ | (14 | ) |  | $ | 1,436 |
Three months ended June 30, 2016 |  |  |  |  |
Equity securities | $ | 4 |  |  | $ | (10 | ) |  | $ | (6 | )
Debt securities | 108 |  |  | (26 | ) |  | 82 |
Total | $ | 112 |  |  | $ | (36 | ) |  | $ | 76 |
Six months ended June 30, 2017 |  |  |  |  |
Equity securities | $ | 2,350 |  |  | $ | — |  |  | $ | 2,350 |
Debt securities | 206 |  |  | (14 | ) |  | 192 |
Total | $ | 2,556 |  |  | $ | (14 | ) |  | $ | 2,542 |
Six months ended June 30, 2016 |  |  |  |  |
Equity securities | $ | 737 |  |  | $ | (10 | ) |  | $ | 727 |
Debt securities | 322 |  |  | (26 | ) |  | 296 |
Total | $ | 1,059 |  |  | $ | (36 | ) |  | $ | 1,023 |
The cumulative balance of credit related other-than-temporary impairment charges, previously recognized as components of earnings, for debt securities held by the Corporation at June 30, 2017 and June 30, 2016 was $10.0 million. There were no other-than-temporary impairment charges recognized for the three and six months ended June 30, 2017 and June 30, 2016. 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, at June 30, 2017 and 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
June 30, 2017 | (in thousands)
State and municipal securities | $ | 214,497 |  |  | $ | (7,125 | ) |  | $ | — |  |  | $ | — |  |  | $ | 214,497 |  |  | $ | (7,125 | )
Corporate debt securities | 4,020 |  |  | (9 | ) |  | 32,148 |  |  | (2,417 | ) |  | 36,168 |  |  | (2,426 | )
Collateralized mortgage obligations | 144,523 |  |  | (3,057 | ) |  | 231,936 |  |  | (6,682 | ) |  | 376,459 |  |  | (9,739 | )
Residential mortgage-backed securities | 896,206 |  |  | (10,974 | ) |  | — |  |  | — |  |  | 896,206 |  |  | (10,974 | )
Commercial mortgage-backed securities | 68,546 |  |  | (335 | ) |  | — |  |  | — |  |  | 68,546 |  |  | (335 | )
Auction rate securities | — |  |  | — |  |  | 97,923 |  |  | (9,487 | ) |  | 97,923 |  |  | (9,487 | )
Total debt securities | $ | 1,327,792 |  |  | $ | (21,500 | ) |  | $ | 362,007 |  |  | $ | (18,586 | ) |  | $ | 1,689,799 |  |  | $ | (40,086 | )
| Less than 12 months |  | 12 months or longer |  | Total
| Estimated Fair Value |  | Unrealized Losses |  | Estimated Fair Value |  | Unrealized Losses |  | Estimated Fair Value |  | Unrealized Losses
December 31, 2016 | (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 | )
Residential mortgage-backed securities | 1,112,947 |  |  | (16,900 | ) |  | — |  |  | — |  |  | 1,112,947 |  |  | (16,900 | )
Commercial mortgage-backed securities | 24,563 |  |  | (537 | ) |  | — |  |  | — |  |  | 24,563 |  |  | (537 | )
Auction rate securities | — |  |  | — |  |  | 97,256 |  |  | (9,959 | ) |  | 97,256 |  |  | (9,959 | )
Total debt securities | $ | 1,563,846 |  |  | $ | (37,122 | ) |  | $ | 390,122 |  |  | $ | (22,713 | ) |  | $ | 1,953,968 |  |  | $ | (59,835 | )
The change in fair value of these securities is attributable to changes in interest rates and not credit quality, and the Corporation does not have the intent to sell and does not believe it will more likely than not be required to sell any of these securities prior to a recovery of their fair value to amortized cost. Therefore, the Corporation does not consider these investments to be other-than-temporarily impaired as of June 30, 2017. As of June 30, 2017, all of the auction rate securities (auction rate certificates, or "ARCs"), were rated above investment grade. All of the loans underlying the ARCs have principal payments which are guaranteed by the federal government. As of June 30, 2017, all ARCs were current and making scheduled interest payments, and based on management’s evaluations, were not subject to any other-than-temporary impairment charges as of June 30, 2017. The Corporation does not have the intent to sell and does not believe it will more likely than not be required to sell any of these securities prior to a recovery of their fair value to amortized cost, which may be at maturity. 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 value of corporate debt securities:
| June 30, 2017 |  | December 31, 2016
| Amortized cost |  | Estimated fair value |  | Amortized cost |  | Estimated fair value
| (in thousands)
Single-issuer trust preferred securities | $ | 43,795 |  |  | $ | 42,478 |  |  | $ | 43,746 |  |  | $ | 39,829 |
Subordinated debt | 29,069 |  |  | 29,697 |  |  | 46,231 |  |  | 46,723 |
Senior debt | 12,035 |  |  | 12,407 |  |  | 18,037 |  |  | 18,433 |
Pooled trust preferred securities | — |  |  | 422 |  |  | — |  |  | 422 |
Corporate debt securities issued by financial institutions | 84,899 |  |  | 85,004 |  |  | 108,014 |  |  | 105,407 |
Other corporate debt securities | 3,988 |  |  | 3,988 |  |  | 4,002 |  |  | 4,002 |
Available for sale corporate debt securities | $ | 88,887 |  |  | $ | 88,992 |  |  | $ | 112,016 |  |  | $ | 109,409 |
Single-issuer trust preferred securities had an unrealized loss of $1.3 million at June 30, 2017. Six of the 19 single-issuer trust preferred securities, with an amortized cost of $11.6 million and an estimated fair value of $10.9 million at June 30, 2017, were rated below investment grade by at least one ratings agency. All of the single-issuer trust preferred securities rated below investment grade were rated "BB" and "Ba". Two single-issuer trust preferred securities with an amortized cost of $3.8 million and an estimated fair value of $2.8 million at June 30, 2017 were not rated by any ratings agency. Based on management’s evaluations, no corporate debt securities were subject to any other-than-temporary impairment charges as of June 30, 2017. The Corporation does not have the intent to sell and does not believe it will more likely than not be required to sell any of these securities prior to a recovery of their fair value to amortized cost, which may be at maturity.

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