Investments - Equity Method Limited Liability Investments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Equity Method Limited Liability Investments, Maximum Exposure to Loss	$ 170.9	$ 161.0
Outstanding Commitments to Fund Equity Method Limited Liability Investments	$ 97.5

Amount of investment in equity method investee and investment in and advance to affiliate.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant