Short-Term Borrowings and Long-Term Debt Repurchase Agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Amount outstanding as of December 31	$ 754,777	$ 617,524	$ 541,317
Customer repurchase agreements
Short-term Debt [Line Items]
Amount outstanding as of December 31	$ 43,500	$ 172,017	$ 195,734
Weighted average interest rate as of December 31	0.25%	0.13%	0.10%
Average amount outstanding during the year	$ 138,198	$ 188,974	$ 184,978
Weighted average interest rate during the year	0.21%	0.12%	0.11%