Convertible Senior Notes - Due 2017 (Details) - USD ($)		1 Months Ended		3 Months Ended
	Jun. 01, 2017	Jun. 30, 2016	Jun. 30, 2012	Mar. 31, 2018	Mar. 31, 2017
Interest expense
Amortization of debt discount				$ 6,745,000	$ 6,973,000
Senior Notes | Convertible Senior Notes Due 2017
Debt Instrument [Line Items]
Principal			$ 275,000,000
Interest rate			1.375%
Proceeds from offering			$ 266,200,000
Repayments of convertible senior notes	$ 55,400,000	$ 323,200,000
Debt repurchase amount		220,000,000
Proceeds from bond hedges and warrants		$ 12,600,000
Issuance of stock upon note conversion (in shares)	819,901
Liability component
Principal			$ 275,000,000
Interest expense
Contractual interest expense				0	189,000
Amortization of debt discount				0	711,000
Total				$ 0	$ 900,000
Effective interest rate of the liability component				0.00%	6.02%