Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Material effect on Company's financial statements	No
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	$ 203
Impact of new guidance on equity if adopted at reporting date	$ 59
Accounting Standards Update 2016-16 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Material effect on Company's financial statements	No
Accounting Standards Update 2017-01 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Material effect on Company's financial statements	No
Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Material effect on Company's financial statements	No
Accounting Standards Update 2017-12 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Material effect on Company's financial statements	No
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Material effect on Company's financial statements	Yes