Stockholders' Equity (Narrative) (Details) - USD ($) shares in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Cash received from option exercises and purchases of shares under the Purchase Plans	$ 5,100,000	$ 4,800,000	$ 4,500,000
Stock-Based Awards
Total intrinsic value of options exercised	7,200,000	9,200,000	8,100,000
Unamortized stock-based compensation expense related to unvested stock awards	$ 48,500,000
Weighted average period of recognition for unrecognized compensation costs (in years)	2 years 18 days
Tax benefit attributable to stock options exercised	$ 500,000	$ 200,000	$ 200,000
DXI Plan
Unamortized stock-based compensation expense related to unvested stock awards	$ 1,300,000
Weighted average period of recognition for unrecognized compensation costs (in years)	2 years 61 days
Employee Stock Purchase Plan
Employee Stock Purchase Plan Shares Issued	0.3	0.4	0.3
Percentage of market value price of common stock under Employee Stock Purchase Plan		85.00%
Maximum contribution percentage amount of employee's base compensation		10.00%