Borrowings (Schedule of Amounts Outstanding and Interest Rates of Short-term Borrowings) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt Disclosure [Abstract]
Balance outstanding at end of year	$ 452,852,000	$ 445,921,000	$ 398,932,000
Average daily balance outstanding	450,009,000	417,585,000	271,281,000
Maximum balance outstanding at any month end	$ 484,288,000	$ 467,811,000	$ 398,932,000
Weighted average interest rate for the year	0.37%	0.19%	0.19%
Weighted average interest rate at end of year	0.46%	0.20%	0.16%