Fair value measurements (Narrative) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($) funds	Dec. 31, 2015 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Trading Securities	$ 8,452,442	$ 7,788,822
Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Trading Securities	$ 8,452,442	7,788,822
Recurring | Hedge fund side pocket
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investment Time Lag Reporting	3 months
Recurring | Hedge funds
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investment Time Lag Reporting	3 months
Other investments number of funds | funds	1
Recurring | Pooled investment
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments number of funds | funds	1
Recurring | Structured securities fund
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments number of funds | funds	2
Recurring | Mutual funds
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments number of funds | funds	2
Recurring | Minimum | Private equity investments
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investment Time Lag Reporting	3 months
Recurring | Maximum | Private equity investments
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investment Time Lag Reporting	6 months
Recurring | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Trading Securities	$ 314,431	245,837
Managed investments
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Trading Securities	6,012,448	5,898,177
Managed investments | Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Trading Securities	6,012,448	5,898,177
Managed investments | Recurring | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Trading Securities	$ 276,396	$ 232,337
Ratio of Level 3 investments to total investments (percent)	4.60%	3.90%