Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Distributions from partnership investments	[1]	$ 74	$ 71
Net cash provided by (used in) investing activities	[1]	(732)	(572)
Accounting Standards Update 2016-15 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Distributions from partnership investments		74
Accounting Standards Update 2016-15 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Distributions from partnership investments			71
Net cash provided by (used in) investing activities			$ (71)
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		230
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 $71 million of cash distributions from partnership earnings from investing to operating activities for the six months ended June 30, 2016. The comparable amount reported in operating activities in 2017 was $74 million. See Note 2 for further discussion.