Fair value measurements Fair Value Measurements (narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Level 1 to Level 2 transfers, Amount	$ 0	$ 0
Fair Value, Level 2 to Level 1 Transfers, Amount		$ 0
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