Stockholders' Equity Stockholders' Equity - Retained Earnings Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of March 31, 2016			$ 967,471
Net income	$ 41,027	$ 156,921	21,414	$ 145,971
Balance as of December 31, 2016	970,498		970,498
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of March 31, 2016			826,449
Repurchase of common stock			12,570
Balance as of December 31, 2016	$ 835,293		$ 835,293