Variable Interest Entities - Textual (Details) - Consolidated Limited Partnership-like Entities [Member] - entity	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
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%