Share-Based Payments - Portfolio Performance Shares Narrative (Details) - Portfolio Performance Shares [Member] |
12 Months Ended |
|---|---|
Dec. 31, 2018 | |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |
| Award requisite service period | 3 years |
| Award vesting period | 5 years |
| Minimum [Member] | |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |
| Number of shares earned as a percentage of initial award | 0.00% |
| Maximum [Member] | |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |
| Number of shares earned as a percentage of initial award | 200.00% |