Debt (Tables)

6 Months Ended

Jun. 30, 2018

Debt Disclosure [Abstract]

Summary of Debt

At June 30, 2018 and December 31, 2017, our long-term debt and interest rates on that debt were as follows (dollars in millions):

	June 30, 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 $0.4 million and    $0.5 million as of June 30, 2018 and December 31, 2017,    respectively, due December 2020		499.6		2.45	%		499.5		2.45	%
3.90% Senior Notes, net of discount of $0.1 million and    $0.2 million as of June 30, 2018 and December 31, 2017,    respectively, due June 2022		399.9		3.90	%		399.8		3.90	%
4.50% Senior Notes, net of discount of $1.1 million and    $1.2 million as of June 30, 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 June 30, 2018 and December 31, 2017,    due September 2024		399.2		3.65	%		399.2		3.65	%
3.40% Senior Notes, net of discount of $1.6 million    as of June 30, 2018 and December 31, 2017,    due December 2027		498.4		3.40	%		498.4		3.40	%
Total		2,496.0		3.64	%		2,645.7		3.80	%
Less current portion		—		—	%		150.0		6.50	%
Less unamortized debt issuance costs		13.9					15.3
Total long-term debt	$	2,482.1		3.64	%	$	2,480.4		3.64	%