(in thousands)	December 31,
	2018			2017
Unsecured revolving lines of credit	$	198,603		$	183,473
4.03% Series A senior notes due in 2018		—			20,000
3.68% Series B senior notes due in 2021		40,000			40,000
3.84% Series C senior notes due in 2022		60,000			60,000
		298,603			303,473
Unamortized debt issuance cost		(39	)		(59	)
	$	298,564		$	303,414

As of December 31, 2018, the future minimum payments under the unsecured revolving lines of credit, 3.68% Series B senior notes due in 2021 and 3.84% Series C senior notes due in 2022 are as follows:

(in thousands)
Year Ended December 31,
2019	$	—
2020		—
2021		238,603
2022		60,000
	$	298,603

In March 2016, the Company renewed its credit agreement with a syndicate of banks (the “Credit Facility”). The five-year facility matures on March 31, 2021 and replaced the Company’s prior $420.0 million unsecured revolving credit facility. The Credit Facility provides for a $420.0 million unsecured revolving credit facility (which may be increased to $620.0 million with $200.0 million of additional commitments), which includes a $25.0 million sublimit for the issuance of standby letters of credit and a $10.0 million sublimit for swingline loans.

In March 2016, the Company entered into a Credit Facility Letter Agreement and a Credit Line Note in favor of MUFG Union Bank, N.A., extending its line of credit facility related to its cash management services (“Sweep Service Facility”) and increasing the facility size from $10.0 million to $12.0 million. The Sweep Service Facility matures on the earlier of March 31, 2021, or the date the Company ceases to utilize MUFG Union Bank, N.A. for its cash management services.

At December 31, 2017, under the Credit Facility and Sweep Service Facility, the Company had unsecured lines of credit that permit it to borrow up to $432.0 million of which $198.6 million was outstanding, and had capacity to borrow up to an additional $233.4 million. The Amended Credit Facility contains financial covenants requiring the Company to not (all defined terms used below not otherwise defined herein have the meaning assigned to such terms in the Amended Credit Facility):

Amounts borrowed under the Credit Facility bear interest at the Company’s option at either: (i) LIBOR plus a defined margin, or (ii) the Agent bank’s prime rate (“base rate”) plus a margin. The applicable margin for each type of loan is measured based upon the Consolidated Leverage Ratio at the end of the prior fiscal quarter and ranges from 1.00% to 1.75% for LIBOR loans and 0% to 0.75% for base rate loans. In addition, the Company pays an unused commitment fee for the portion of the $420.0 million credit facility that is not used. These fees are based upon the Consolidated Leverage Ratio and range from 0.15% to 0.30%. As of December 31, 2018 and 2017, the applicable margins were 1.25% for LIBOR based loans, 0.25% for base rate loans and 0.20% for unused fees. Amounts borrowed under the Sweep Service Facility are based upon the MUFG Union Bank, N.A. base rate plus an applicable margin and an unused commitment fee for the portion of the $12.0 million facility not used. The applicable base rate margin and unused commitment fee rates for the Sweep Service Facility are the same as for the Amended Credit Facility. The following information relates to the lines of credit for each of the following periods:

(dollar amounts in thousands)	Year Ended December 31,
	2018			2017
Maximum amount outstanding	$	228,500		$	215,732
Average amount outstanding	$	204,534		$	199,499
Weighted average interest rate, during the period		3.36	%		2.64	%
Prime interest rate, end of period		5.50	%		4.50	%

On April 21, 2011, the Company entered into a Note Purchase and Private Shelf Agreement (the “Note Purchase Agreement”) with Prudential Investment Management, Inc. (“PIM”), The Prudential Insurance Company of America and Prudential Retirement Insurance and Annuity Company (collectively, the “Purchaser”), pursuant to which the Company agreed to sell an aggregate principal amount of $100.0 million of its 4.03% Series A Senior Notes (the “Series A Senior Notes”) to the Purchaser. The Series A Senior Notes were an unsecured obligation of the Company, due on April 21, 2018. Interest on these notes was due semi-annually in arrears and the principal was due in five equal annual installments, with the first payment due on April 21, 2014. In addition, the Note Purchase Agreement allowed for the issuance and sale of additional senior notes to the Purchaser (the “Shelf Notes”) in the aggregate principal amount of $100.0 million, to mature no more than 12 years after the date of original issuance thereof, to have an average life of no more than 10 years and to bear interest on the unpaid balance. The final $20.0 million principal payment under the Series A Senior Notes was made in April 2018 with no amount remaining outstanding as of December 31, 2018. At December 31, 2017, the principal balance outstanding under the Series A Senior Notes was $20 million.

On March 17, 2014, the Company issued and sold to the Purchasers a $40.0 million aggregate principal amount of its 3.68% Series B Senior Notes (the “Series B Senior Notes”) pursuant to the terms of the Note Purchase Agreement, as amended. The Series B Senior Notes are an unsecured obligation of the Company, bear interest at a rate of 3.68% per annum and mature on March 17, 2021. Interest on the Series B Senior Notes is payable semi-annually beginning on September 17, 2014 and continuing thereafter on March 17 and September 17 of each year until maturity. The principal balance is due when the notes mature in 2021. The full net proceeds from the Series B Senior Notes were used for working capital and other general corporate purposes. At December 31, 2018 and 2017, the principal balance outstanding under the Series B Senior Notes was $40.0 million.

On November 5, 2015, the Company issued and sold to the Purchasers a $60.0 million aggregate principal amount of its 3.84% Series C Senior Notes (the “Series C Senior Notes”) pursuant to the terms of the Note Purchase Agreement, as amended. The Series C Senior Notes are an unsecured obligation of the Company, bear interest at a rate of 3.84% per annum and mature on November 5, 2022. Interest on the Series C Senior Notes is payable semi-annually beginning on May 5, 2016 and continuing thereafter on November 5 and May 5 of each year until maturity. The principal balance is due when the notes mature in 2022. The full net proceeds from the Series C Senior Notes were used to reduce the outstanding balance on the Company’s revolving credit line. At December 31, 2018 and 2017, the principal balance outstanding under the Series C Senior Notes was $60.0 million.

Among other restrictions, the Note Purchase Agreement, under which the Series A Senior Notes, Series B Senior Notes and Series C Senior Notes were sold, contains financial covenants requiring the Company to not (all defined terms used below not otherwise defined herein have the meaning assigned to such terms in the Note Purchase Agreement):

At December 31, 2018, the Company was in compliance with each of the aforementioned covenants. There are no anticipated trends that the Company is aware of that would indicate non-compliance with these covenants, though, significant deterioration in the Company’s financial performance could impact its ability to comply with these covenants.

On February 9, 2016, the Company entered into an amendment to the Note Purchase Agreement (“2016 Amendment”) with the Purchaser. Pursuant to the 2016 Amendment, (i) the issuance period for the shelf notes to be issued and sold pursuant to the Note Purchase Agreement is extended until the earlier of February 9, 2019 or the termination of the issuance and sale of the shelf notes upon the 30 days’ prior notice of either PIM or the Company, and (ii) the definition of the “Available Facility Amount,” which is the aggregate amount of the shelf notes that may be authorized for purchase pursuant to the Note Purchase Agreement was amended to equal a formula based on: $250 million, minus the aggregate principal amount of the shelf notes then outstanding and purchased pursuant to the Note Purchase Agreement, minus the shelf notes accepted by the Company for purchase, but not yet purchased, by the Purchaser pursuant to the Note Purchase Agreement; provided, however, the aggregate amount of the shelf notes purchased by any corporation or other entity controlling, controlled by, or under common control with, PIM shall not exceed $200 million.