Long-Term Debt - Interest Rates (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Mar. 31, 2017
1.875% Senior Notes
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	$ 3,389	$ 3,971
Debt instrument, interest rate	1.875%	1.875%	1.875%
4.15% Senior Notes
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	$ 3,128	$ 3,372
Debt instrument, interest rate	4.15%	4.15%
4.50% Senior Notes
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	$ 740	$ 891
Debt instrument, interest rate	4.50%	4.50%	4.50%
5.45% Senior Notes
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	$ 4,082	$ 4,159
Debt instrument, interest rate	5.45%	5.45%