Fair Value Measurements (Carrying Value and Fair Value of Debt) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Mar. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value	$ 2,025,505	$ 1,998,772
3.25% senior notes due October 2015 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value	249,981	249,962
Long-term debt, fair value	250,945	252,520
2.95% senior notes due June 2016 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value	249,936	249,918
Long-term debt, fair value	253,708	254,953
1.65% senior notes due February 2018 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value	324,715	324,688
Long-term debt, fair value	323,105	323,921
2.375% senior notes due February 2020 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value	274,802	274,791
Long-term debt, fair value	271,816	274,821
2.90% senior notes due November 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value	249,794	249,787
Long-term debt, fair value	241,933	249,028
3.65% senior notes due July 2024 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value	299,476	299,462
Long-term debt, fair value	$ 299,130	$ 310,500