Fair Value Measurements (Carrying Value and Fair Value of Debt) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Mar. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value	$ 2,206,465	$ 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	0	249,962
Long-term debt, fair value	0	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	249,953	249,918
Long-term debt, fair value	252,810	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	324,742	324,688
Long-term debt, fair value	323,242	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	274,813	274,791
Long-term debt, fair value	272,861	274,821
3.05% senior notes due August 2020 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value	399,251	0
Long-term debt, fair value	406,564	0
2.90% senior notes due November 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value	249,801	249,787
Long-term debt, fair value	241,253	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	299,491	299,462
Long-term debt, fair value	$ 296,961	$ 310,500