Stockholders' Equity (Narrative) (Details) - USD ($)	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Stock-Based Awards
Total intrinsic value of options exercised	$ 9,000,000	$ 7,200,000	$ 9,200,000
Unamortized stock-based compensation expense related to unvested stock awards	$ 63,900,000
Weighted average period of recognition for unrecognized compensation costs (in years)	2 years 180 days
Employee Stock Purchase Plan
Unamortized stock-based compensation expense related to unvested stock awards	$ 500,000
Weighted average period of recognition for unrecognized compensation costs (in years)	180 days
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%