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Borrowings (Schedule of Amounts Outstanding and Interest Rates of Short-term Borrowings) (Details) (USD $)
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12 Months Ended | ||
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Dec. 31, 2014
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Dec. 31, 2013
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Dec. 31, 2012
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| Debt Disclosure [Abstract] | |||
| Balance outstanding at end of year | $ 445,921,000 | $ 398,932,000 | $ 192,681,000 |
| Average daily balance outstanding | 417,585,000 | 271,281,000 | 261,335,000 |
| Maximum balance outstanding at any month end | $ 467,811,000 | $ 398,932,000 | $ 346,786,000 |
| Weighted average interest rate for the year | 0.19% | 0.19% | 0.21% |
| Weighted average interest rate at end of year | 0.20% | 0.16% | 0.18% |