Stockholders' Equity - Retained Earnings (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Sep. 30, 2018	Mar. 31, 2018
Increase (Decrease) in Stockholders' Equity
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Retained Earnings
Increase (Decrease) in Stockholders' Equity
Balance as of March 31, 2018	$ 785,871
Net income	43,965
Repurchases of common stock	(134,724)
Impact from adoption of ASUs, net of tax		$ 468
Balance as of September 30, 2018	695,580
2015 And 2017 Stock Repurchase Plans
Increase (Decrease) in Stockholders' Equity
Repurchases of common stock	$ (134,735)
Shares repurchased (in shares)	1,151,260