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Short-term Borrowings (Summary of Short-Term Borrowings) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Federal Funds Purchased |  |  |
Short-term Debt [Line Items] |  |  |
Average balance | $ 447,137 | $ 589,223 | $ 705,054
Year-end balance | 399,820 | 414,207 | 464,166
Maximum month-end outstanding | $ 568,490 | $ 695,083 | $ 1,228,125
Average rate for the year | 1.06% | 0.52% | 0.26%
Average rate at year-end | 1.48% | 0.73% | 0.50%
Securities Sold Under Agreements to Repurchase |  |  |
Short-term Debt [Line Items] |  |  |
Average balance | $ 578,666 | $ 425,452 | $ 370,097
Year-end balance | 656,602 | 453,053 | 338,133
Maximum month-end outstanding | $ 743,684 | $ 528,024 | $ 524,191
Average rate for the year | 0.72% | 0.08% | 0.06%
Average rate at year-end | 0.64% | 0.08% | 0.09%
Trading Liabilities |  |  |
Short-term Debt [Line Items] |  |  |
Average balance | $ 685,891 | $ 771,039 | $ 733,189
Year-end balance | 638,515 | 561,848 | 566,019
Maximum month-end outstanding | $ 896,943 | $ 874,076 | $ 866,005
Average rate for the year | 2.26% | 1.95% | 2.18%
Average rate at year-end | 2.22% | 2.46% | 2.41%
Other Short-term Borrowings |  |  |
Short-term Debt [Line Items] |  |  |
Average balance | $ 554,502 | $ 198,440 | $ 164,951
Year-end balance | 2,626,213 | 83,177 | 137,861
Maximum month-end outstanding | $ 2,626,213 | $ 792,736 | $ 339,468
Average rate for the year | 1.28% | 0.67% | 0.67%
Average rate at year-end | 1.44% | 0.96% | 0.82%

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