Short-term Funding, Composition (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Abstract]
Federal funds purchased	$ 141,950	$ 208,150
Securities sold under agreements to repurchase	182,865	300,197
Federal funds purchased and securities sold under agreements to repurchase	324,815	508,347
FHLB advances	284,000	482,000
Commercial paper	67,467	101,688
Other short-term funding	351,467	583,688
Total short-term funding	$ 676,282	$ 1,092,035