Accumulated Other Comprehensive Income (Schedule of Amounts Recognized in Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands |
3 Months Ended | 12 Months Ended | ||||
|---|---|---|---|---|---|---|
Mar. 31, 2019 |
Mar. 31, 2018 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Jan. 01, 2019 |
Dec. 31, 2018 |
|
| AOCI Attributable to Parent, Net of Tax [Roll Forward] | ||||||
| Beginning Balance | $ 1,047,383 | $ 990,549 | $ 990,549 | |||
| Cumulative effect of accounting change | $ 0 | |||||
| Beginning Balance | 1,047,383 | 990,549 | 990,549 | $ 990,549 | 1,047,383 | |
| Ending Balance | 1,026,659 | 1,047,383 | 990,549 | |||
| Unrealized Income on Derivative Instruments [Member] | ||||||
| AOCI Attributable to Parent, Net of Tax [Roll Forward] | ||||||
| Beginning Balance | 6,100 | 3,710 | 3,710 | 840 | ||
| Cumulative effect of accounting change | 103 | |||||
| Beginning Balance | 6,100 | 3,710 | 3,710 | 840 | $ 6,203 | $ 6,100 |
| Other comprehensive (loss) income before reclassification | (2,955) | 4,446 | 3,208 | 1,484 | ||
| Amount of (income) loss reclassified from accumulated other comprehensive income to net income (effective portion) | (656) | 129 | (818) | 1,386 | ||
| Other comprehensive income (loss) | (3,611) | 4,575 | 2,390 | 2,870 | ||
| Ending Balance | $ 2,592 | $ 8,285 | $ 6,100 | $ 3,710 | ||