Summary of Significant Accounting Policies Cash, Cash Equivalent, and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 19, 2018	Dec. 31, 2017	Dec. 22, 2017	Mar. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Senior Notes		$ 100,000		$ 300,000
Cash and cash equivalents	$ 143,951		$ 176,274		$ 187,928
Restricted Cash, Current	51				0
Restricted Cash, Noncurrent	1,466		311,010		0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	145,468		487,335		$ 187,928	$ 133,565
Senior Notes [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Senior Notes	$ 400,000		$ 300,000
Debt Instrument, Interest Rate, Stated Percentage			5.00%