Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Distributions from partnership investments	[1]	$ 114	$ 105
Net cash provided by (used in) investing activities	[1]	(1,100)	(1,694)
Accounting Standards Update 2016-15 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Distributions from partnership investments		114
Accounting Standards Update 2016-15 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Distributions from partnership investments			105
Net cash provided by (used in) investing activities			$ (105)
Accounting Standards Update 2016-01 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Current carrying value of certain limited partnership interests to be reported at fair value upon adoption of ASU 2016-01		220
Impact of new guidance on equity if adopted at reporting date		$ 50
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Material effect on Company's financial statements		No

[1] As required in adopting Accounting Standard Update ("ASU") 2016-15, the Company retrospectively reclassified $105 million of cash distributions from partnership earnings from investing to operating activities for the nine months ended September 30, 2016. The comparable amount reported in operating activities in 2017 was $114 million. See Note 2 for further discussion.