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Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands | Jun. 30, 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 | $ 458,800 |
Available-for-Sale Amortized Cost, After One Year but Within Five Years | 1,490,200 |
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years | 218,200 |
Available-for-Sale Amortized Cost, After Ten Years | 59,000 |
Amortized Cost | 2,226,200 | $ 2,229,300
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity |  |
Available-for-Sale Estimated Fair Value, Within One Year | 450,500 |
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years | 1,457,500 |
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years | 212,200 |
Available-for-Sale Estimated Fair Value, After Ten Years | 57,800 |
Available-for-Sale, Estimated Fair Value | 2,178,000 | 2,208,700
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount |  |
Held-to-Maturity Amortized Cost, Within One Year | 76,300 |
Held-to-Maturity Amortized Cost, After One Year but Within Five Years | 221,400 |
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years | 110,700 |
Held-to-Maturity Amortized Cost, After Ten Years | 32,500 |
Amortized Cost | 440,900 | 484,500
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity |  |
Held-to-Maturity Estimated Fair Value, Within One Year | 75,300 |
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years | 215,700 |
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years | 110,500 |
Held-to-Maturity Estimated Fair Value, After Ten Years | 31,300 |
Held-to-Maturity, Estimated Fair Value | 432,800 | $ 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 | 107,200 |
Fair value of investment securities callable after one year but within five years | $ 104,800 |

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