12.

Debt

At March 31, 2018 and December 31, 2017, our long-term debt and interest rates on that debt were as follows (dollars in millions):

	March 31, 2018					December 31, 2017
	Amount		Interest Rate			Amount		Interest Rate
6.50% Senior Notes due March 2018	$	—		—	%	$	150.0		6.50	%
2.45% Senior Notes, net of discount of $0.5 million as of March 31, 2018 and December 31, 2017, due December 2020		499.5		2.45	%		499.5		2.45	%
3.90% Senior Notes, net of discount of $0.2 million as of March 31, 2018 and December 31, 2017, due June 2022		399.8		3.90	%		399.8		3.90	%
4.50% Senior Notes, net of discount of $1.1 million and $1.2 million as of March 31, 2018 and December 31, 2017, respectively, due November 2023		698.9		4.50	%		698.8		4.50	%
3.65% Senior Notes, net of discount of $0.8 million as of March 31, 2018 and December 31, 2017, due September 2024		399.2		3.65	%		399.2		3.65	%
3.40% Senior Notes, net of discounts of $1.6 million as of March 31, 2018 and December 31, 2017 due December 2027		498.4		3.40	%		498.4		3.40	%
Total		2,495.8		3.64	%		2,645.7		3.80	%
Less current portion		—		—	%		150.0		6.50	%
Less unamortized debt issuance costs		14.6					15.3
Total long-term debt	$	2,481.2		3.64	%	$	2,480.4		3.64	%

During the three months ended March 31, 2018, we used cash on hand to repay debt outstanding of $150.0 million under the 6.50% Senior Notes due March 2018. For the three months ended March 31, 2018 and 2017, cash payments for interest were $12.5 million and $18.0 million, respectively.

Included in interest expense, net, are amortization of treasury lock settlements and amortization of financing costs. For both the three months ended March 31, 2018 and 2017, amortization of treasury lock settlements was $1.4 million. For the three months ended March 31, 2018 and 2017, amortization of financing costs was $0.7 million and $0.5 million, respectively.

At March 31, 2018, we have $2,495.8 million of fixed-rate senior notes outstanding. The fair value of our fixed-rate debt was estimated to be $2,515.0 million. The difference between the book value and fair value is due to the difference between the period-end market interest rate and the stated rate of our fixed-rate debt. We estimated the fair value of our fixed-rate debt using quoted market prices (Level 2 inputs) within the fair value hierarchy, which is further defined in Note 2, Summary of Significant Accounting Policies, of the Notes to Consolidated Financial Statements in "Part II, Item 8. Financial Statements and Supplementary Data" of our 2017 Annual Report on Form 10-K.

For more information on our long-term debt and interest rates on that debt, see Note 9, Debt, of the Notes to Consolidated Financial Statements in "Part II, Item 8. Financial Statements and Supplementary Data" of our 2017 Annual Report on Form 10-K.