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Investment Securities | 3 Months Ended
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Mar. 31, 2018
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)
March 31, 2018 |  |  |  |  |  |  |
U.S. Government sponsored agency securities | $ | 21,105 |  |  | $ | 40 |  |  | $ | (183 | ) |  | $ | 20,962 |
State and municipal securities | 413,632 |  |  | 2,247 |  |  | (9,780 | ) |  | 406,099 |
Corporate debt securities | 101,367 |  |  | 2,482 |  |  | (1,894 | ) |  | 101,955 |
Collateralized mortgage obligations | 695,840 |  |  | 236 |  |  | (16,376 | ) |  | 679,700 |
Residential mortgage-backed securities | 1,084,582 |  |  | 3,006 |  |  | (33,422 | ) |  | 1,054,166 |
Commercial mortgage-backed securities | 231,378 |  |  | 15 |  |  | (4,501 | ) |  | 226,892 |
Auction rate securities | 107,410 |  |  | — |  |  | (4,361 | ) |  | 103,049 |
Total | $ | 2,655,314 |  |  | $ | 8,026 |  |  | $ | (70,517 | ) |  | $ | 2,592,823 |
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Value
| (in thousands)
December 31, 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 |
Securities carried at $1.8 billion at March 31, 2018 and December 31, 2017, were pledged as collateral to secure public and trust deposits and customer repurchase agreements. The amortized cost and estimated fair values of debt securities as of March 31, 2018, 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 |  | $ | 14,055 |  |  | $ | 14,063 |
Due from one year to five years |  | 36,871 |  |  | 36,986 |
Due from five years to ten years |  | 119,069 |  |  | 119,529 |
Due after ten years |  | 473,519 |  |  | 461,487 |
|  | 643,514 |  |  | 632,065 |
Residential mortgage-backed securities(1) |  | 1,084,582 |  |  | 1,054,166 |
Commercial mortgage-backed securities(1) |  | 231,378 |  |  | 226,892 |
Collateralized mortgage obligations(1) |  | 695,840 |  |  | 679,700 |
Total |  | $ | 2,655,314 |  |  | $ | 2,592,823 |
(1) Maturities for mortgage-backed securities and collateralized mortgage obligations are dependent upon the interest rate environment and prepayments on the underlying loans.
The following table presents information related to the gross realized gains on the sales of equity and debt securities:
| Gross Realized Gains
Three months ended March 31, 2018 | (in thousands)
Equity securities | $ | 9 |
Debt securities | 10 |
Total | $ | 19 |
Three months ended March 31, 2017 |
Equity securities | $ | 1,045 |
Debt securities | 61 |
Total | $ | 1,106 |
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 March 31, 2018 and March 31, 2017 was $11.5 million. There were no other-than-temporary impairment charges recognized for the three months ended March 31, 2018 and March 31, 2017. 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 March 31, 2018 and 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
March 31, 2018 | (in thousands)
U.S. Government sponsored agency securities | $ | 12,499 |  |  | $ | (183 | ) |  | $ | — |  |  | $ | — |  |  | $ | 12,499 |  |  | $ | (183 | )
State and municipal securities | 163,345 |  |  | (3,132 | ) |  | 114,024 |  |  | (6,648 | ) |  | 277,369 |  |  | (9,780 | )
Corporate debt securities | 9,466 |  |  | (130 | ) |  | 28,219 |  |  | (1,764 | ) |  | 37,685 |  |  | (1,894 | )
Collateralized mortgage obligations | 456,200 |  |  | (7,184 | ) |  | 176,076 |  |  | (9,192 | ) |  | 632,276 |  |  | (16,376 | )
Residential mortgage-backed securities | 527,502 |  |  | (13,611 | ) |  | 471,703 |  |  | (19,811 | ) |  | 999,205 |  |  | (33,422 | )
Commercial mortgage-backed securities | 196,887 |  |  | (3,884 | ) |  | 21,539 |  |  | (617 | ) |  | 218,426 |  |  | (4,501 | )
Auction rate securities | — |  |  | — |  |  | 103,049 |  |  | (4,361 | ) |  | 103,049 |  |  | (4,361 | )
Total | $ | 1,365,899 |  |  | $ | (28,124 | ) |  | $ | 914,610 |  |  | $ | (42,393 | ) |  | $ | 2,280,509 |  |  | $ | (70,517 | )
| 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, 2017 | (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 | )
The Corporation’s collateralized mortgage obligations and mortgage-backed securities have contractual terms that generally do not permit the issuer to settle the securities at a price less than the amortized cost of the investment. 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 March 31, 2018. As of March 31, 2018, all of the auction rate securities (auction rate certificates, or "ARCs"), were rated above investment grade. Based on management’s evaluations, none of the ARCs were subject to any other-than-temporary impairment charges for the three months ended March 31, 2018. 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 values of corporate debt securities:
| March 31, 2018 |  | December 31, 2017
| Amortized cost |  | Estimated fair value |  | Amortized cost |  | Estimated fair value
| (in thousands)
Single-issuer trust preferred securities | $ | 31,352 |  |  | $ | 30,513 |  |  | $ | 31,335 |  |  | $ | 30,703 |
Subordinated debt | 54,011 |  |  | 54,357 |  |  | 49,013 |  |  | 49,533 |
Senior debt | 12,029 |  |  | 12,245 |  |  | 12,031 |  |  | 12,392 |
Pooled trust preferred securities | — |  |  | 865 |  |  | — |  |  | 707 |
Corporate debt securities issued by financial institutions | 97,392 |  |  | 97,980 |  |  | 92,379 |  |  | 93,335 |
Other corporate debt securities | 3,975 |  |  | 3,975 |  |  | 3,974 |  |  | 3,974 |
Available for sale corporate debt securities | $ | 101,367 |  |  | $ | 101,955 |  |  | $ | 96,353 |  |  | $ | 97,309 |
Single-issuer trust preferred securities had an unrealized loss of $839,000 at March 31, 2018. Four of the 17 single-issuer trust preferred securities, with an amortized cost of $4.9 million and an estimated fair value of $4.7 million at March 31, 2018, 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 either "BB" or "Ba". Two single-issuer trust preferred securities with an amortized cost of $3.8 million and an estimated fair value of $3.1 million at March 31, 2018 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 for the three months ended March 31, 2018. 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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