Fair value measurements Fair Value Measurements (narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Level 1 to Level 2 transfers, Amount	$ 0	$ 0	$ 0	$ 0
Fair Value, Level 2 to Level 1 Transfers, Amount	0	$ 0	$ 0	$ 0
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