SHORT-TERM BORROWINGS

Selected information for FHLB advances and other short-term borrowings is as follows:



(Dollar amounts in millions)	2017			2016			2015
Federal Home Loan Bank advances
Average amount outstanding	$	2,657		$	37		$	—
Average rate	1.13		%	0.59		%	—		%
Highest month-end balance	3,750			750			—
Year-end balance	3,600			500			—
Average rate on outstandings at year-end	1.44		%	0.75		%	—		%
Other short-term borrowings, year-end balances
Federal funds purchased	927			106			111
Security repurchase agreements	354			196			206
Securities sold, not yet purchased	95			25			30
Total Federal Home Loan Bank advances and other short-term borrowings	$	4,976		$	827		$	347

ZB, N.A. may borrow from the FHLB under their lines of credit that are secured under blanket pledge arrangements. ZB, N.A. maintained unencumbered collateral with carrying amounts adjusted for the types of collateral pledged, equal to at least 100% of the outstanding advances. At December 31, 2017, the amount available for FHLB advances was approximately $8.9 billion. ZB, N.A. may also borrow from the Federal Reserve based on the amount of collateral pledged to a Federal Reserve Bank. At December 31, 2017, the amount available for additional Federal Reserve borrowings was approximately $5.8 billion.

Federal funds purchased and security repurchase agreements generally mature in less than 30 days. Our participation in security repurchase agreements is on an overnight or term basis (e.g., 30 or 60 days). ZB, N.A. executes overnight repurchase agreements with sweep accounts in conjunction with a master repurchase agreement. When this occurs, securities under the Bank’s control are pledged and interest is paid on the collected balance of the customers’ accounts. For the non-sweep overnight and term repurchase agreements, securities are transferred to the applicable counterparty. The counterparty, in certain instances, is contractually entitled to sell or repledge securities accepted as collateral. Of the total security repurchase agreements at December 31, 2017, $31 million were overnight and $323 million were term.