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Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity |  |
Available-for-Sale Amortized Cost, Within One Year | $ 433,000 |
Available-for-Sale Amortized Cost, After One Year but Within Five Years | 1,445,700 |
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years | 278,800 |
Available-for-Sale Amortized Cost, After Ten Years | 167,500 |
Amortized Cost | 2,325,000 | $ 2,229,300
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity |  |
Available-for-Sale Estimated Fair Value, Within One Year | 426,300 |
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years | 1,417,800 |
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years | 272,200 |
Available-for-Sale Estimated Fair Value, After Ten Years | 164,400 |
Available-for-Sale, Estimated Fair Value | 2,280,700 | 2,208,700
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount |  |
Held-to-Maturity Amortized Cost, Within One Year | 61,400 |
Held-to-Maturity Amortized Cost, After One Year but Within Five Years | 247,200 |
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years | 121,000 |
Held-to-Maturity Amortized Cost, After Ten Years | 32,700 |
Amortized Cost | 462,300 | 484,500
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity |  |
Held-to-Maturity Estimated Fair Value, Within One Year | 61,200 |
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years | 241,600 |
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years | 120,800 |
Held-to-Maturity Estimated Fair Value, After Ten Years | 31,900 |
Held-to-Maturity, Estimated Fair Value | 455,500 | $ 483,300
Callable Within One Year |  |
Available-for-sale Securities and Held-to-maturity Securities |  |
Amortized cost of investment securities callable after one year but within five years | 147,500 |
Fair value of investment securities callable after one year but within five years | $ 144,400 |

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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing in the next rolling twelve months following the latest balance sheet. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments and fair value hedge accounting adjustments. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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