Summary Debt Outstanding Based On Stated Maturities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Debt [Line Items]
Long-term Debt, Gross	$ 2,040,525	$ 2,052,270
Less: Deferred debt issue costs	6,401	6,754
Total debt	2,034,124	2,045,516
3.90% September 2010 Senior Notes Due Fiscal 2018
Debt [Line Items]
Long-term Debt, Gross	44,500	44,500
3.95% September 2010 Senior Notes Due Fiscal 2018
Debt [Line Items]
Long-term Debt, Gross	25,000	25,000
4.12% September 2010 Senior Notes Due Fiscal 2019
Debt [Line Items]
Long-term Debt, Gross	25,000	25,000
4.17% September 2010 Senior Notes Due Fiscal 2019
Debt [Line Items]
Long-term Debt, Gross	25,000	25,000
4.33% September 2010 Senior Notes Due Fiscal 2020
Debt [Line Items]
Long-term Debt, Gross	50,000	50,000
4.51% September 2010 Senior Notes Due Fiscal 2021
Debt [Line Items]
Long-term Debt, Gross	100,000	100,000
4.56% September 2010 Senior Notes Due Fiscal 2021
Debt [Line Items]
Long-term Debt, Gross	65,000	65,000
4.61% September 2010 Senior Notes Due Fiscal 2023
Debt [Line Items]
Long-term Debt, Gross	48,000	48,000
4.06% August 2011 Senior Notes Due Fiscal 2019
Debt [Line Items]
Long-term Debt, Gross	50,000	50,000
4.54% August 2011 Senior Notes Due Fiscal 2022
Debt [Line Items]
Long-term Debt, Gross	65,000	65,000
4.64% August 2011 Senior Notes Due Fiscal 2022
Debt [Line Items]
Long-term Debt, Gross	50,000	50,000
4.26% September 2013 senior notes due fiscal 2021
Debt [Line Items]
Long-term Debt, Gross	123,000	123,000
5.01% September 2013 senior notes due fiscal 2024
Debt [Line Items]
Long-term Debt, Gross	250,000	250,000
5.01% September 2013 senior notes due fiscal 2024
Debt [Line Items]
Long-term Debt, Gross	127,000	127,000
Norwegian Kroner Denominated Notes Due Fiscal Two Thousand Twenty Five
Debt [Line Items]
Long-term Debt, Gross	14,864	17,500
Norwegian Kroner Denominated Notes Due Fiscal Two Thousand Twenty Six
Debt [Line Items]
Long-term Debt, Gross	26,167	30,207
United States Dollar Denominated Borrowing Agreement Due Fiscal Two Thousand Twenty Seven
Debt [Line Items]
Long-term Debt, Gross	24,573	27,030
United States Dollar Denominated Borrowing Agreement Due Fiscal Two Thousand Twenty Eight
Debt [Line Items]
Long-term Debt, Gross	27,421	30,033
Term Loan Facility Due Twenty Twenty
Debt [Line Items]
Term loan	300,000	300,000
Revolving Line Of Credit Due Fiscal Twenty Twenty
Debt [Line Items]
Revolving line of credit	$ 600,000	$ 600,000