Common Stock Transactions (Details) $ / shares in Units, $ in Thousands			1 Months Ended		3 Months Ended
	Mar. 18, 2016 USD ($) $ / shares shares	Mar. 17, 2016	Mar. 31, 2016 USD ($) director item $ / shares shares	Mar. 31, 2015 USD ($) $ / shares shares	Mar. 31, 2016 USD ($) director $ / shares	Mar. 31, 2015 USD ($) $ / shares
Net proceeds from the sale of common stock in an underwritten public offering					$ 154,972	$ 179,950
Accredited investors
Number of directors | director			3		3
Number of placement agents used | item			0
Common Stock
Number of shares issued in private placement | shares	4,404,658
Common stock issue price, in an underwritten public offering (in dollars per share) | $ / shares	$ 35.19
Proceeds from issuance of private placement	$ 155,000
Common Stock | Underwritten public offering
Number of shares issued | shares				3,755,000
Common stock issue price, in an underwritten public offering (in dollars per share) | $ / shares				$ 51.00		$ 51.00
Gross proceeds from the sale of common stock in an underwritten public offering				$ 191,500
Net proceeds from the sale of common stock in an underwritten public offering				$ 179,800
Common Stock | Accredited investors
Number of shares issued in private placement | shares			4,404,658
Common stock issue price, in an underwritten public offering (in dollars per share) | $ / shares			$ 35.19		$ 35.19
Proceeds from issuance of private placement			$ 155,000
Number of days for calculating weighted average price per share		10 days