Fair value measurements (Narrative) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($) funds	Dec. 31, 2015 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Trading Securities	$ 8,280,766	$ 7,788,822
Other investments, at fair value	359,526	336,856
Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Trading Securities	$ 8,280,766	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	$ 293,049	245,837
Managed investments
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Trading Securities	5,942,679	5,898,177
Other investments, at fair value	359,526	336,856
Managed investments | Hedge funds
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments, at fair value	18,625	20,980
Managed investments | Private equity investments
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments, at fair value	73,306	63,771
Managed investments | Investment funds
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments, at fair value	203,871	188,721
Managed investments | Mutual funds
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments, at fair value	7,464	7,483
Managed investments | Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Trading Securities	$ 5,942,679	$ 5,898,177
Ratio of Level 3 investments to total investments (percent)	4.30%	3.90%
Other investments, at fair value	$ 359,526	$ 336,856
Managed investments | Recurring | Hedge funds
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments, at fair value	18,625	20,980
Managed investments | Recurring | Private equity investments
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments, at fair value	73,306	63,771
Managed investments | Recurring | Investment funds
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments, at fair value	203,871	188,721
Managed investments | Recurring | Mutual funds
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments, at fair value	7,464	7,483
Managed investments | Recurring | Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Trading Securities	255,531	232,337
Other investments, at fair value	0	0
Managed investments | Recurring | Level 3 | Hedge funds
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments, at fair value	0	0
Managed investments | Recurring | Level 3 | Private equity investments
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments, at fair value	0	0
Managed investments | Recurring | Level 3 | Investment funds
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments, at fair value	0	0
Managed investments | Recurring | Level 3 | Mutual funds
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments, at fair value	$ 0	$ 0