Fair value measurements - Significant unobservable inputs (Details) $ in Thousands	Mar. 31, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($) $ / shares
Significant Unobservable Inputs
Equity securities	$ 366,600	$ 405,200
Derivative assets	11,700	11,400
Derivative liabilities	(20,400)	(9,500)
Contingent consideration	(28,300)	(28,200)
Level 3 Inputs
Significant Unobservable Inputs
Equity securities	0	0
Derivative assets	10,700	10,100
Derivative liabilities	(19,600)	(9,300)
Contingent consideration	(28,300)	(28,200)
Level 3 Inputs | Share price of recent transaction | Private equity securities
Significant Unobservable Inputs
Equity securities	32,500	32,500
Level 3 Inputs | Share price of recent transaction | Preferred stock
Significant Unobservable Inputs
Equity securities	$ 17,500	$ 17,500
Level 3 Inputs | Share price of recent transaction | Purchase share price | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input | $ / shares	40.6	40.6
Level 3 Inputs | Share price of recent transaction | Purchase price | Preferred stock
Significant Unobservable Inputs
Equity securities, measurement input | $ / shares	7.74	7.74
Level 3 Inputs | Purchase price of recent transaction | Loan participation
Significant Unobservable Inputs
Equity securities	$ 19,900	$ 20,000
Equity securities, measurement input		20,000
Level 3 Inputs | Purchase price of recent transaction | Private debt instrument
Significant Unobservable Inputs
Debt securities, trading	6,200	$ 7,200
Level 3 Inputs | Purchase price of recent transaction | Private equity securities
Significant Unobservable Inputs
Equity securities	5,100	300
Level 3 Inputs | Purchase price of recent transaction | Preferred stock
Significant Unobservable Inputs
Equity securities	1,900	12,200
Level 3 Inputs | Purchase price of recent transaction | Private equity funds
Significant Unobservable Inputs
Equity securities	1,000	5,100
Level 3 Inputs | Purchase price of recent transaction | Preferred stock
Significant Unobservable Inputs
Equity securities	700	4,800
Level 3 Inputs | Purchase price of recent transaction | Private equity securities
Significant Unobservable Inputs
Equity securities	$ 300
Level 3 Inputs | Purchase price of recent transaction | Private equity securities
Significant Unobservable Inputs
Equity securities		1,000
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Private debt instrument
Significant Unobservable Inputs
Private debt instrument, measurement input, value		9,000
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input, value		300
Equity securities, measurement input | $ / shares	7.74
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Preferred stock
Significant Unobservable Inputs
Equity securities, measurement input, value	$ 1,900	$ 12,200
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Private equity funds
Significant Unobservable Inputs
Equity securities, measurement input, value	0
Equity securities, measurement input | $ / shares		7.74
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Preferred stock
Significant Unobservable Inputs
Equity securities, measurement input, value	700	$ 4,800
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input, value	300
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input | $ / shares		10.0
Level 3 Inputs | Multiple of GAAP book value | Private equity securities
Significant Unobservable Inputs
Equity securities	13,300	$ 14,200
Level 3 Inputs | Multiple of GAAP book value | Book value multiple | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input | $ / shares		0.9
Level 3 Inputs | Third party appraisal | Weather derivatives
Significant Unobservable Inputs
Derivative assets		$ 7,000
Derivative liabilities	(13,900)
Level 3 Inputs | Third party appraisal | Currency forwards
Significant Unobservable Inputs
Derivative assets	1,700	(2,700)
Level 3 Inputs | Third party appraisal | Foreign currency swaps
Significant Unobservable Inputs
Derivative assets	2,400
Derivative liabilities		(3,600)
Level 3 Inputs | Third party appraisal | Currency forwards
Significant Unobservable Inputs
Derivative liabilities		(5,700)
Level 3 Inputs | Third party appraisal | Broker quote | Weather derivatives
Significant Unobservable Inputs
Derivative assets, measurement input, value		7,000
Derivative liabilities, measurement input, value	(13,900)
Level 3 Inputs | Third party appraisal | Broker quote | Currency forwards
Significant Unobservable Inputs
Derivative assets, measurement input, value	(1,700)	2,700
Level 3 Inputs | Third party appraisal | Broker quote | Foreign currency swaps
Significant Unobservable Inputs
Derivative assets, measurement input, value	(2,400)
Derivative liabilities, measurement input, value		(3,600)
Level 3 Inputs | Third party appraisal | Broker quote | Currency forwards
Significant Unobservable Inputs
Derivative liabilities, measurement input, value		(5,700)
Level 3 Inputs | Option pricing model
Significant Unobservable Inputs
Equity securities	900	400
Level 3 Inputs | Option pricing model | Strike price
Significant Unobservable Inputs
Equity securities, measurement input, value	200	200
Level 3 Inputs | External valuation model
Significant Unobservable Inputs
Contingent consideration	(28,300)	(28,200)
Level 3 Inputs | External valuation model | Discounted future payments
Significant Unobservable Inputs
Contingent consideration, measurement input, value	$ (28,300)	$ (28,200)
Minimum | Level 3 Inputs | Purchase price of recent transaction | Comparable yields | Loan participation
Significant Unobservable Inputs
Equity securities, measurement input	0.04
Minimum | Level 3 Inputs | Purchase price of recent transaction | Discount yield | Private debt instrument
Significant Unobservable Inputs
Debt Securities, Trading, Measurement Input	0.1187
Minimum | Level 3 Inputs | Multiple of GAAP book value | Book value multiple | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input | $ / shares	0.73
Maximum | Level 3 Inputs | Purchase price of recent transaction | Comparable yields | Loan participation
Significant Unobservable Inputs
Equity securities, measurement input	0.07
Maximum | Level 3 Inputs | Purchase price of recent transaction | Discount yield | Private debt instrument
Significant Unobservable Inputs
Debt Securities, Trading, Measurement Input	0.1232
Maximum | Level 3 Inputs | Multiple of GAAP book value | Book value multiple | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input | $ / shares	0.91
Median | Level 3 Inputs | Purchase price of recent transaction | Comparable yields | Loan participation
Significant Unobservable Inputs
Equity securities, measurement input	0.06
Median | Level 3 Inputs | Purchase price of recent transaction | Discount yield | Private debt instrument
Significant Unobservable Inputs
Debt Securities, Trading, Measurement Input	0.1208
Median | Level 3 Inputs | Multiple of GAAP book value | Book value multiple | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input | $ / shares	0.82