Borrowings - Federal Funds Purchased (Narrative) (Details) - Federal funds purchased - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Average balance during the year	$ 317.4	$ 481.9	$ 377.1
Weighted average interest rate	0.57%	0.33%	0.30%
Maximum amount outstanding at any month end	$ 370.0	$ 660.0	$ 455.0