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Investment Securities (Tables) | 12 Months Ended
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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 |

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- Definition Tabular disclosure of the amount of other than temporary impairment (OTTI) related to credit losses recognized in earnings including: (a) the beginning balance of the amount related to credit losses on debt securities held by the investor at the beginning of the period for which a portion of an OTTI was recognized in other comprehensive income; (b) additions for the amount related to the credit loss for which an OTTI was not previously recognized; (c) reductions for securities sold during the period (realized); (d) reductions for securities for which the amount previously recognized in other comprehensive income was recognized in earnings because the investor intends to sell the security or more likely than not will be required to sell the security before recovery of its amortized cost basis; (e) additional increases to the amount related to the credit loss for which an OTTI was previously recognized when the investor does not intend to sell the security and it is not more likely than not that the investor will be required to sell the security before recovery of its amortized cost basis; (f) reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security; and (g) the ending balance of the amount related to credit losses on debt securities held by the entity at the end of the period for which a portion of an OTTI was recognized in other comprehensive income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27340-111563 + Details
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