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Borrowings Borrowings - Short-term Debt (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Debt Disclosure [Abstract]      
Outstanding at December 31, $ 150,580 $ 243,351 $ 209,730
Maximum month-end outstanding balance 266,439 352,603 303,521
Average daily outstanding balance $ 134,425 $ 262,674 $ 215,685
Average rate during the year 0.76% 0.54% 0.28%
Average rate at year end 1.21% 0.58% 0.37%