12. SHORT-TERM BORROWINGS

The bank was a member of the Federal Home Loan Bank of Seattle until its merger with the Federal Home Loan Bank of Des Moines on June 1, 2015. We are now a member of the Federal Home Loan Bank of Des Moines (the "FHLB") and maintained a $1.50 billion line of credit, of which $1.47 billion remained available as of December 31, 2017. At December 31, 2017, short-term borrowings under this arrangement totaled $32.0 million. At December 31, 2016, short-term borrowings under this arrangement totaled $135.0 million.

At December 31, 2017 and 2016, our bank had additional unused borrowings available at the Federal Reserve discount window of $73.0 million and $63.7 million, respectively. As of December 31, 2017 and 2016, certain commercial real estate and commercial loans with a carrying value totaling $129.2 million and $129.9 million, respectively, were pledged as collateral on our line of credit with the Federal Reserve discount window. The Federal Reserve does not have the right to sell or repledge these loans.

Interest expense on short-term borrowings were $0.2 million, $0.6 million and $0.3 million in 2017, 2016 and 2015, respectively.

A summary of our short-term borrowings as of December 31, 2017, 2016 and 2015 is as follows:



	Year Ended December 31,
	2017			2016			2015
	(Dollars in thousands)
Amount outstanding at December 31	$	32,000		$	135,000		$	69,000
Average amount outstanding during year	15,531			110,928			92,045
Highest month-end balance during year	69,000			226,000			157,000
Weighted average interest rate on balances outstanding at December 31	1.63		%	0.74		%	0.35		%
Weighted average interest rate during year	1.18		%	0.52		%	0.28		%