Fair value measurements (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) funds	Dec. 31, 2014 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	$ 7,788,822	$ 7,381,128
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	7,788,822	7,381,128
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	$ 245,837	50,248
Mutual funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of funds | funds	2
Hedge funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of funds | funds	1
Investment time lag reporting	3 months
Investment funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment time lag reporting	1 month
Pooled investment | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of funds | funds	1
Structured securities fund | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of funds | funds	1
Hedge fund side pocket | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment time lag reporting	3 months
Minimum | Private equity investments | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment time lag reporting	3 months
Maximum | Private equity investments | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment time lag reporting	6 months
Managed investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	$ 5,898,177	6,110,044
Managed investments | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	$ 5,898,177	$ 6,110,044
Ratio of Level 3 investments to total investments (percent)	3.90%	0.50%
Managed investments | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	$ 232,337	$ 32,748