Summary of Significant Accounting Policies - Impairment of Cost Basis Investments (Details) - Private Equity Partnership That Invests in Renewable Energy Companies - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2017	Jun. 30, 2017
Schedule of Cost-method Investments [Line Items]
Carrying amount of investment		$ 11.0
Corporate & Other
Schedule of Cost-method Investments [Line Items]
Other-than-temporary impairment of investment	$ 7.0

Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 325
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75025870&loc=d3e40691-111596

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 325
-SubTopic 20
-Section 35
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=75027002&loc=d3e40346-111594

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant