Stockholders' Equity - Stock Repurchase Plan, Retained Earnings Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2017
Increase (Decrease) in Stockholders' Equity
Net income	$ 86,341	$ 41,027	$ 93,779	$ 21,414
Retained Earnings
Increase (Decrease) in Stockholders' Equity
Beginning balance			819,589
Net income			93,779
Repurchase of common stock			(24,683)
Cumulative unrecognized excess tax benefit					$ 1,365
Ending balance	$ 890,050		$ 890,050