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Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity |  |
Available-for-sale amortized cost, within one year | $ 450,700 |
Available-for-sale amortized cost, after one year but within five years | 1,422,700 |
Available-for-sale amortized cost, after five years but within ten years | 277,600 |
Available-for-sale amortized cost, after ten years | 78,300 |
Amortized Cost | 2,229,300 | $ 1,625,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity |  |
Available-for-sale estimated fair value, within one year | 447,500 |
Available-for-sale estimated fair value, after one year but within five years | 1,409,300 |
Available-for-sale estimated fair value, after ten years | 274,200 |
Available-for-sale estimated fair value, after five years but within ten years | 77,700 |
Available-for-Sale, Estimated Fair Value | 2,208,712 | 1,611,698
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount |  |
Held-to-maturity amortized cost, within one year | 76,100 |
Held-to-maturity amortized cost, after one year but within five years | 266,200 |
Held-to-maturity amortized cost, after five years but within ten years | 111,700 |
Held-to-maturity amortized cost, after ten years | 30,500 |
Amortized Cost | 484,494 | 512,770
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity |  |
Held-to-maturity estimated fair value, within one year | 76,700 |
Held-to-maturity estimated fair value, after one year but within five years | 263,100 |
Held-to-maturity estimated fair value, after five years but within ten years | 113,300 |
Held-to-maturity estimated fair value, after ten years | 30,200 |
Estimated Fair Value | 483,349 | 513,273
Available-for-sale Securities and Held-to-maturity Securities |  |
Securities sold under agreements to repurchase | 2,087,700 | 1,400,100
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value | 2,062,600 | 1,388,200
Callable within one year |  |
Available-for-sale Securities and Held-to-maturity Securities |  |
Investment securities primarily classified as available-for-sale, amortized costs, after one year but within five years | 133,200 |
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years | 132,900 |
Mortgage Backed Securities, Variable Rate |  |
Available-for-sale Securities and Held-to-maturity Securities |  |
Available-for-sale securities, amortized cost basis | $ 247,900 | $ 66,200

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