Debt (Details) - USD ($)	Apr. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2014
Senior Notes 3.2 Percent Due October 1, 2026
Debt
Debt, interest rate (as a percent)		3.20%	3.20%
Senior Notes 6 Percent Due April 15, 2017
Debt
Debt, interest rate (as a percent)		6.00%
Debt, face amount		$ 200,000,000
Senior Notes 1.8 Percent Due September 25, 2017
Debt
Debt, interest rate (as a percent)		1.80%	1.80%
Debt, face amount		$ 300,000,000
Senior Notes 4.625 Percent Due November 1, 2020
Debt
Debt, interest rate (as a percent)		4.625%	4.625%
Debt, face amount				$ 400,000,000
Revolving Credit Agreement
Debt
Debt, face amount	$ 200,000,000