General (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Available for sale equity securities, included in other comprehensive income	$ 659	$ 509
Limited partnership interests	5,982	5,814
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Available for sale equity securities, included in other comprehensive income	659
EMA limited partnerships
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Limited partnership interests	4,690	$ 4,530
EMA limited partnerships | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Limited partnership interests	$ 194