Fair value measurements Fair Value Measurements (narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Realized Gain Losses As A Result of Annuitization Changes		$ 94
Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Notice Period For Redemption For Alternative Investments Investment Funds	5 days
Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Notice Period For Redemption For Alternative Investments Investment Funds	120 days