Summary of Significant Accounting Policies Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 22, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Line Items]
Senior Notes		$ 300,000
Cash and cash equivalents	$ 176,274		$ 133,565	$ 131,018
Restricted Cash, Current	51		0	0
Restricted Cash, Noncurrent	311,010		0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	487,335		133,565	$ 131,018	$ 112,371
Senior Notes [Member]
Cash and Cash Equivalents [Line Items]
Senior Notes	$ 300,000		$ 0
Debt Instrument, Interest Rate, Stated Percentage	5.00%