Short-Term Borrowings and Long-Term Debt Repurchase Agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Amount outstanding as of December 31	$ 497,663	$ 329,719	$ 1,258,629
Customer repurchase agreements
Short-term Debt [Line Items]
Amount outstanding as of December 31	$ 111,496	$ 158,394	$ 175,621
Weighted average interest rate as of December 31	0.15%	0.13%	0.12%
Average amount outstanding during the year	$ 161,093	$ 197,432	$ 186,851
Weighted average interest rate during the year	0.10%	0.10%	0.11%