Variable Interest Entities - Textual (Details) - Consolidated Limited Partnership-like Entities [Member] - entity |
6 Months Ended | 12 Months Ended |
---|---|---|
Jun. 30, 2018 |
Dec. 31, 2017 |
|
Variable Interest Entity [Line Items] | ||
Number of consolidated limited partnership-like entities | 10 | 10 |
Minimum [Member] | ||
Variable Interest Entity [Line Items] | ||
Ownership interest in consolidated entities (percent) | 6.80% | |
Maximum [Member] | ||
Variable Interest Entity [Line Items] | ||
Ownership interest in consolidated entities (percent) | 99.50% |