Summary of Significant Accounting Policies (Reconciliation of Cash, Cash Equivalents and Restricted Cash) (Details) - USD ($) $ in Thousands |
Mar. 31, 2019 |
Dec. 31, 2018 |
Mar. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|---|---|---|---|---|---|---|
| Accounting Policies [Abstract] | ||||||
| Cash and cash equivalents | $ 73,727 | $ 68,360 | $ 76,734 | $ 74,803 | $ 50,446 | $ 31,262 |
| Restricted cash | 11,614 | 11,167 | 12,885 | 10,944 | 6,463 | 1,927 |
| Cash, cash equivalents and restricted cash | $ 85,341 | $ 79,527 | $ 89,619 | $ 85,747 | $ 56,909 | $ 33,189 |