Summary of Significant Accounting Policies (Details 6) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity securities	$ 509	$ 276
Limited partnership interests	5,814	4,874
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity securities	509
Equity-method limited partnerships
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Limited partnership interests	4,530	$ 3,720
Equity-method limited partnerships | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Limited partnership interests	$ 178