Summary of Significant Accounting Policies Cash, Cash Equivalent, and Restricted Cash (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and cash equivalents	$ 110,828	$ 143,951	$ 120,221
Current restricted cash included in other current assets	51	51	51
Long-term restricted cash	2,086	1,466	2,056
Total cash, cash equivalents, and restricted cash	112,965	145,468	$ 122,328	$ 487,335
Proceeds from borrowings	$ 30,000	705,938
Improper Netting Within Financing Activities On Cash Flow Statement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Proceeds from borrowings		$ 150,000