Short-Term Borrowings and Long-Term Debt Repurchase Agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Amount outstanding as of December 31	$ 617,524	$ 541,317	$ 497,663
Customer repurchase agreements
Short-term Debt [Line Items]
Amount outstanding as of December 31	$ 172,017	$ 195,734	$ 111,496
Weighted average interest rate as of December 31	0.13%	0.10%	0.15%
Average amount outstanding during the year	$ 188,974	$ 184,978	$ 161,093
Weighted average interest rate during the year	0.12%	0.11%	0.10%