Derivative Instruments (Interest Rate Caps and Collars) (Details) $ in Millions |
Dec. 31, 2018
USD ($)
|
|---|---|
| Interest Rate Cap | |
| Derivative [Line Items] | |
| Notional amount of derivative | $ 254.9 |
| Interest Rate Collar | |
| Derivative [Line Items] | |
| Notional amount of derivative | $ 649.9 |